Add checks for whether gdk-pixbuf exists before using avatar set

tests/unittests/xmpp/stub_avatar.c

@@ -8,8 +8,11 @@ avatar_get_by_nick(const char* nick)
 {
     return TRUE;
 }
+
+#ifdef HAVE_PIXBUF
 gboolean
 avatar_set(const char* path)
 {
-    return FALSE;
+    return TRUE;
 }
+#endif