Added SDL_GetErrorMsg() to get the error message in a thread-safe way

src/SDL_error.c

@@ -39,8 +39,6 @@ SDL_LookupString(const char *key)
 
 /* Public functions */
 
-static char *SDL_GetErrorMsg(char *errstr, int maxlen);
-
 int
 SDL_SetError(SDL_PRINTF_FORMAT_STRING const char *fmt, ...)
 {
@@ -192,7 +190,7 @@ main(int argc, char *argv[])
 /* This function has a bit more overhead than most error functions
    so that it supports internationalization and thread-safe errors.
 */
-static char *
+char *
 SDL_GetErrorMsg(char *errstr, int maxlen)
 {
     SDL_error *error;