Move ID generation to xmpp folder

create_unique_id() was changed to use UUIDs instead of a counter in the
last commit. Since now it depends on connection_create_uuid() which is
in the xmpp subfolder the function should also be moved there.

Renamed it to connection_create_stanza_id() and moved it to
src/xmpp/connection.c.

Discussion happened in https://github.com/boothj5/profanity/pull/1010

src/common.h

@@ -93,7 +93,6 @@ char* release_get_latest(void);
 gboolean release_is_new(char *found_version);
 
 char* p_sha1_hash(char *str);
-char* create_unique_id(char *prefix);
 
 char* get_file_or_linked(char *loc, char *basedir);
 char* strip_arg_quotes(const char *const input);