Visibility of non-API functions changed to "internal"

The visibility for functions marked as LUAI_FUNC was changed from
"hidden" to "internal". These functions cannot be called from
outside the Lua kernel, and "internal" visibility offers more
chances for optimizations.

luaconf.h

@@ -277,7 +277,7 @@
 */
 #if defined(__GNUC__) && ((__GNUC__*100 + __GNUC_MINOR__) >= 302) && \
     defined(__ELF__)		/* { */
-#define LUAI_FUNC	__attribute__((visibility("hidden"))) extern
+#define LUAI_FUNC	__attribute__((visibility("internal"))) extern
 #else				/* }{ */
 #define LUAI_FUNC	extern
 #endif				/* } */