cdrom: Rename "stat" to "cdr_stat"

"stat" is a C function from <sys/stat.h>. While libpcsxcore/cdrom.c does
not include this header, some of the headers it does include may
actually include it themselves (depending on the C library).

Work around possible name conflicts by just renaming the offending
variable.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>