µhttpd-util: use #pragma to silence warning about nonliteral pattern

This is safe, because we're taking a pattern which was already marked with
_printf_ and appending a literal string.

src/journal-remote/microhttpd-util.c

@@ -103,7 +103,10 @@ int mhd_respondf(struct MHD_Connection *connection,
         errno = -error;
         fmt = strjoina(format, "\n");
         va_start(ap, format);
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
         r = vasprintf(&m, fmt, ap);
+#pragma GCC diagnostic pop
         va_end(ap);
 
         if (r < 0)