Include stdlib.h in avatar.c

We use malloc() and free().

Fix https://github.com/profanity-im/profanity/issues/1241

src/xmpp/avatar.c

@@ -35,6 +35,7 @@
 
 #include <glib.h>
 #include <stdio.h>
+#include <stdlib.h>
 #include <errno.h>
 #include <sys/stat.h>