Bug 1638870 - add unicode/uniset.h to system headers; r=firefox-build-system-reviewers,rstewart

I haven't been able to confirm that this would actually fix
`--with-system-icu`, but since the new regexp
engine (`js/src/new-regexp`) explicitly includes this file and this file
isn't already present in system-headers...it seems very likely that this
change will fix `--with-system-icu`.

Differential Revision: https://phabricator.services.mozilla.com/D75809

config/system-headers.mozbuild

@@ -1315,6 +1315,7 @@ if CONFIG['MOZ_SYSTEM_ICU']:
         'unicode/ulistformatter.h',
         'unicode/uenum.h',
         'unicode/uformattedvalue.h',
+        'unicode/uniset.h',
         'unicode/unistr.h',
         'unicode/unorm.h',
         'unicode/unum.h',