s3/rpc_server: mdssvc: suppress compiler warnings from glib headers

s3/rpc_server: mdssvc: suppress compiler warnings from glib headers

Several glib headers produce cast-qual warnings, eg:

/usr/include/glib-2.0/gio/gliststore.h: In function ‘G_LIST_STORE’:
/usr/include/glib-2.0/gio/gliststore.h:36:382: error: cast discards
‘const’ qualifier from pointer target type [-Werror=cast-qual]
/usr/include/glib-2.0/gio/gliststore.h: In function ‘G_IS_LIST_STORE’:
/usr/include/glib-2.0/gio/gliststore.h:36:550: error: cast discards
‘const’ qualifier from pointer target type [-Werror=cast-qual]
cc1: all warnings being treated as errors

This break compiling with --picky-developer, so lets suppress the
warning for glibs in order to see our own --picky-developer compiler
diagnostics.

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Volker Lendecke <vl@samba.org>
Autobuild-Date(master): Tue Apr 26 04:44:44 CEST 2016 on sn-devel-144