bug 1471281: build: make asan depend on --help; r=froydnj

The asan definition needs to depend on --help for the --enable-geckodriver
flag to depend on it through geckodriver_default.

build/moz.configure/toolchain.configure

@@ -1416,8 +1416,8 @@ add_old_configure_assignment('MOZ_LTO_LDFLAGS', lto.ldflags)
 js_option('--enable-address-sanitizer', help='Enable Address Sanitizer')
 
 
-@depends_if('--enable-address-sanitizer')
-def asan(value):
+@depends_if('--enable-address-sanitizer', '--help')
+def asan(value, _):
     return True