Bug 1492121 - Suppress dlopen leak triggered by next patch - r=njn

Valgrind report:
```
TEST-UNEXPECTED-FAIL | valgrind-test | 192 bytes in 3 blocks are definitely lost at malloc / dl_open_worker / _dl_catch_error / _dl_open
==2561== 192 bytes in 3 blocks are definitely lost in loss record 348 of 399
==2561==    at 0x4C2B240: malloc+112 (vg_replace_malloc.c:298)
==2561==    by 0x4012919: dl_open_worker+1977 (dl-open.c:457)
==2561==    by 0x400DD55: _dl_catch_error+101 (dl-error.c:178)
==2561==    by 0x4011CC9: _dl_open+185 (dl-open.c:633)
==2561==    by 0x5051F65: dlopen_doit+101 (dlopen.c:67)
==2561==    by 0x400DD55: _dl_catch_error+101 (dl-error.c:178)
==2561==    by 0x50522EB: _dlerror_run+123 (dlerror.c:164)
==2561==    by 0x5051EE0: dlopen@@GLIBC_2.2.5+48 (dlopen.c:88)
==2561==    by 0x1148FC: GetLibHandle (xpcom/glue/standalone/nsXPCOMGlue.cpp:86)
==2561==    by 0x1148FC: ReadDependentCB (xpcom/glue/standalone/nsXPCOMGlue.cpp:136)
==2561==    by 0x1148FC: XPCOMGlueLoad (xpcom/glue/standalone/nsXPCOMGlue.cpp:306)
==2561==    by 0x1148FC: mozilla::GetBootstrap(char const*, mozilla::LibLoadingStrategy)+572 (xpcom/glue/standalone/nsXPCOMGlue.cpp:374)
==2561==    by 0x114213: InitXPCOMGlue(mozilla::LibLoadingStrategy)+131 (browser/app/nsBrowserApp.cpp:223)
==2561==    by 0x113E6B: main+219 (browser/app/nsBrowserApp.cpp:284)
```

Nothing due directly to this patch, so it is likely a dlopen issue.

Differential Revision: https://phabricator.services.mozilla.com/D33904

--HG--
extra : moz-landing-system : lando

build/valgrind/x86_64-pc-linux-gnu.sup

@@ -195,7 +195,7 @@
    ...
 }
 {
-  _g_io_module_get_default
+   _g_io_module_get_default
    Memcheck:Leak
    ...
    fun:g_io_module_new
@@ -203,6 +203,20 @@
    fun:_g_io_modules_ensure_loaded
    fun:_g_io_module_get_default
 }
+{
+   dlopen leak triggered by bug 1492121
+   Memcheck:Leak
+   match-leak-kinds: definite
+   fun:malloc
+   fun:dl_open_worker
+   fun:_dl_catch_error
+   fun:_dl_open
+   fun:dlopen_doit
+   fun:_dl_catch_error
+   fun:_dlerror_run
+   fun:dlopen@@GLIBC_2.2.5
+   ...
+}
 
 ###################################
 #  Leaks in short lived precesses #
@@ -864,7 +878,7 @@
 #    at 0x4C2CAEE: operator new(unsigned long) (vg_replace_malloc.c:332)
 #    by 0xF6AB737: std::vector<unsigned short, std::allocator<unsigned short>
 #    by 0xFCD0FDE: ots::ots_post_parse(ots::Font*, unsigned char const*, unsi
-#    by 0xFCCA3D9: (anonymous namespace)::ProcessGeneric(ots::OpenTypeFile*, 
+#    by 0xFCCA3D9: (anonymous namespace)::ProcessGeneric(ots::OpenTypeFile*,
 #    by 0xFCCB17E: (anonymous namespace)::ProcessTTF(ots::OpenTypeFile*, ots:
 #    by 0xFCCBA54: ots::OTSContext::Process(ots::OTSStream*, unsigned char co
 #    by 0xE7D7C8D: gfxUserFontEntry::SanitizeOpenTypeData(unsigned char const