Fix header cleaup fallout when using system pcre

Using the internal pcre has the side effect of exposing gregex.c to
glib.h.  When we use the system one, we lose that, so we need to
explicitly include the things we use (glist, gatomic, etc..)

glib/gregex.c

@@ -32,6 +32,10 @@
 #include "gtypes.h"
 #include "gregex.h"
 #include "glibintl.h"
+#include "glist.h"
+#include "gmessages.h"
+#include "gstrfuncs.h"
+#include "gatomic.h"
 
 /**
  * SECTION:gregex