meson: add -fvisibility=hidden explicitly to selected targets

Don't use it project-wide for building everything. Otherwise
symbols for shared modules won't be exposed, e.g. in the
resourceplugin used by the gio resource unit test.

gio/meson.build

@@ -4,6 +4,8 @@ gio_c_args = [
   '-DGIO_MODULE_DIR="@0@"'.format(glib_giomodulesdir),
 ]
 
+gio_c_args += glib_hidden_visibility_args
+
 # FIXME: Install empty glib_giomodulesdir
 
 gnetworking_h_conf = configuration_data()
@@ -747,6 +749,7 @@ gio_tool_sources = [
   'gio-tool-tree.c',
 ]
 
+# FIXME: are all these HAVE_CONFIG_H needed/used here?
 executable('gio', gio_tool_sources,
   install : true,
   c_args : ['-DHAVE_CONFIG_H=1'] + gio_c_args,

gio/xdgmime/meson.build

@@ -13,4 +13,4 @@ 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' ] + glib_hidden_visibility_args)