Fix warning/error C4013 (implicit declarations)

-Include gthread.h in gregex.c as g_once_init_enter and g_once_init_leave
 are used.
-Define prototype for g_thread_DllMain in gthreadprivate.h for Windows

glib/gregex.c

@@ -36,6 +36,7 @@
 #include "gmessages.h"
 #include "gstrfuncs.h"
 #include "gatomic.h"
+#include "gthread.h"
 
 /**
  * SECTION:gregex

glib/gthreadprivate.h

@@ -67,6 +67,10 @@ struct _GPrivate
 G_GNUC_INTERNAL void g_private_init (GPrivate       *key,
                                      GDestroyNotify  notify);
 
+#ifdef G_OS_WIN32
+G_GNUC_INTERNAL void g_thread_DllMain (void);
+#endif
+
 G_END_DECLS
 
 #endif /* __G_THREADPRIVATE_H__ */