build: Add GLib requirement

For consistency with other components and for the GLib macros set by
XDT_CHECK_PACKAGE when building with xfce4-dev-tools >= 4.17.1.

src/task-manager-bsd.c

@@ -16,6 +16,10 @@
  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  */
 
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
 #include <stdlib.h>
 #include <err.h>
 #include <sys/types.h>