gio: Use g_strerror() instead of strerror()

This marginally improves thread safety, and marginally improves
consistency.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=733821

gio/tests/gsubprocess-testprog.c

@@ -25,7 +25,7 @@ write_all (int           fd,
       gssize bytes_written = write (fd, buf, len);
       if (bytes_written < 0)
 	g_error ("Failed to write to fd %d: %s",
-		 fd, strerror (errno));
+		 fd, g_strerror (errno));
       buf += bytes_written;
       len -= bytes_written;
     }