GitLab: Fix post_gitlab_snippet headers

This has been broken since 98df89e when it was changed to use
hub->http_post

Other than Content and Content_Type k:v pairs, hub->http_request()
presumes all other k:v pairs are headers.

By calling with with headers => { foo=> bar }, the actual arguments into
http_client->do_request() were

headers => { headers => { foo => bar } }
instead of
headers => { foo => bar }

lib/Synergy/Reactor/GitLab.pm

@@ -1064,10 +1064,7 @@ async sub mr_report ($self, $who, $arg = {}) {
 async sub post_gitlab_snippet ($self, $payload) {
   my $res = await $self->hub->http_post(
     $self->api_uri . '/v4/snippets',
-    headers => {
-      'PRIVATE-TOKEN' => $self->api_token,
-    },
-
+    'PRIVATE-TOKEN' => $self->api_token,
     Content_Type => 'application/json',
     Content      => encode_json($payload),
   );