output: use GTimer instead of time_t for reopen after failure

time() is not a monotonic timer, and MPD might get confused by clock
skews.  clock_gettime() provides a monotonic clock, but is not
portable to non-POSIX systems (i.e. Windows).  This patch uses GLib's
GTimer API, which aims to be portable.

src/output_command.c

@@ -38,7 +38,6 @@ audio_output_enable_index(unsigned idx)
 
 	ao = audio_output_get(idx);
 
-	ao->reopen_after = 0;
 	ao->enabled = true;
 	idle_add(IDLE_OUTPUT);