defs: add a new WP_PRIVATE_API function annotation

This is to mark private functions that are exposed in public headers.
These functions will not be exported from the library and will
generate a warning when client code is trying to use them.

lib/wp/defs.h

@@ -27,4 +27,12 @@
 # endif
 #endif
 
+#ifndef WP_PRIVATE_API
+# ifdef BUILDING_WP
+#  define WP_PRIVATE_API
+# else
+#  define WP_PRIVATE_API __attribute__ ((deprecated ("Private API")))
+# endif
+#endif
+
 #endif