xdgmime: Disable -Wsign-conversion warning inside xdgmime

The code wasn’t written for it, and we don’t want to diverge from
upstream for this, so disable -Wsign-conversion in case it was set for
the overall GLib build.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3405

gio/xdgmime/meson.build

@@ -9,11 +9,20 @@ xdgmime_sources = files(
   'xdgmimeparent.c',
 )
 
+# glib enables various warnings which the xdgmime code wasn’t designed to
+# work with
+extra_xdgmime_args = cc.get_supported_arguments([
+  '-Wno-sign-conversion',
+])
+
 xdgmime_lib = static_library('xdgmime',
   sources : xdgmime_sources,
   include_directories : [configinc],
   pic : true,
-  c_args : [ '-DHAVE_CONFIG_H',
-             '-DXDG_PREFIX=_gio_xdg' ],
+  c_args : [
+    '-DHAVE_CONFIG_H',
+    '-DXDG_PREFIX=_gio_xdg',
+    extra_xdgmime_args,
+  ],
   gnu_symbol_visibility : 'hidden',
 )