ui-repolist: fix redefinition of _XOPEN_SOURCE

Previously, ui-repolist.c set _GNU_SOURCE and then included a standard
library before including <git-compat-util.h>.  This was a problem,
because <git-compat-util.h> redefined _XOPEN_SOURCE, which is set
automatically by glibc when _GNU_SOURCE is set.  However,
<git-compat-util.h> already sets _GNU_SOURCE and includes both
<string.h> and <time.h>, so there is no need to define _GNU_SOURCE or
include either header within ui-repolist.c.

Signed-off-by: Mark Lodato <lodatom@gmail.com>