locale: Make sure C++ implementations (Haiku!) use C linkage.

src/locale/SDL_syslocale.h

@@ -24,6 +24,14 @@
 
 #include "SDL_locale.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 extern void SDL_SYS_GetPreferredLocales(char *buf, size_t buflen);
 
+#ifdef __cplusplus
+}
+#endif
+
 /* vi: set ts=4 sw=4 expandtab: */