Revert "use designated initializers for yajl_callbacks struct"

This reverts commit 705b43294a.

This commit broke i3bar for some users.

i3-msg/main.c

@@ -119,11 +119,17 @@ static int reply_map_key_cb(void *params, const unsigned char *keyVal, unsigned
 }
 
 yajl_callbacks reply_callbacks = {
-    .yajl_boolean = reply_boolean_cb,
-    .yajl_string = reply_string_cb,
-    .yajl_start_map = reply_start_map_cb,
-    .yajl_map_key = reply_map_key_cb,
-    .yajl_end_map = reply_end_map_cb,
+    NULL,
+    &reply_boolean_cb,
+    NULL,
+    NULL,
+    NULL,
+    &reply_string_cb,
+    &reply_start_map_cb,
+    &reply_map_key_cb,
+    &reply_end_map_cb,
+    NULL,
+    NULL
 };
 
 int main(int argc, char *argv[]) {