[libsoup/fix-error-string-is-not-a-literal-string] Fix Clang compilation error 'format string is not a string literal'

[Diego Pino <dpino src gnome org>]

commit 3614c2f52ca2b22ff9ebb0409be218298600d364
Author: Diego Pino Garcia <dpino igalia com>
Date:   Wed May 19 16:30:25 2021 +0800

    Fix Clang compilation error 'format string is not a string literal'

 libsoup/http2/soup-client-message-io-http2.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/libsoup/http2/soup-client-message-io-http2.c b/libsoup/http2/soup-client-message-io-http2.c
index 90edcefa..af6968c4 100644
--- a/libsoup/http2/soup-client-message-io-http2.c
+++ b/libsoup/http2/soup-client-message-io-http2.c
@@ -187,6 +187,7 @@ state_to_string (SoupHTTP2IOState state)
         }
 }
 
+__attribute__((__format__ (__printf__, 3, 0)))
 static void
 h2_debug (SoupClientMessageIOHTTP2   *io,
           SoupHTTP2MessageData       *data,