xfree86: up the path name size to 512 in xf86MatchDriverFromFiles

./hw/xfree86/common/xf86pciBus.c: In function ‘xf86MatchDriverFromFiles’:
../hw/xfree86/common/xf86pciBus.c:1330:52: warning: ‘snprintf’ output may be
truncated before the last format character [-Wformat-truncation=]
             snprintf(path_name, sizeof(path_name), "%s/%s", ^~~~~~~
../hw/xfree86/common/xf86pciBus.c:1330:13: note: ‘snprintf’ output between 2

dirent->d_name is 256, so sprintf("%s/%s") into a 256 buffer gives us:

and 257 bytes into a destination of size 256

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit 96af794d)