mtk: Include config.h in C files

Make these pull the general config, might be handy in the
future.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3230>

mtk/meson.build

@@ -2,7 +2,10 @@ mtk_includesubdir = pkgname / 'mtk'
 mtk_includedir = includedir / mtk_includesubdir
 
 mtk_includepath = include_directories('.', 'mtk')
-mtk_includes = [mtk_includepath]
+mtk_includes = [
+  mtk_includepath,
+  top_includepath,
+]
 
 mtk_c_args = [
   '-DMTK_SYSCONFDIR="@0@"'.format(prefix / sysconfdir),

mtk/mtk/mtk-rectangle.c

@@ -19,6 +19,8 @@
  * License along with this library. If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include "config.h"
+
 #include "mtk/mtk-rectangle.h"
 
 

mtk/mtk/mtk-x11-errors.c

@@ -23,6 +23,8 @@
  * Mutter X error handling
  */
 
+#include "config.h"
+
 #include "mtk/mtk-x11-errors.h"
 
 #include <errno.h>