nostrdb: bolt11: move utf8_check into local function.

It isn't actually in the CCAN module (though it probably should be!).
So it breaks when we update.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: William Casarin <jb55@jb55.com>

nostrdb/src/util.h

@@ -28,6 +28,5 @@ static inline char *strdupn(const char *src, size_t size) {
 	dest[size] = '\0';
 	return dest;
 }
-
 #endif // NDB_UTIL_H