Fix a subtle bug exposed by recent wal_sync_method rearrangements.

Formerly, the default value of wal_sync_method was determined inside xlog.c,
but now it is determined inside guc.c.  guc.c was reading xlogdefs.h
without having read <fcntl.h>, leading to wrong determination of
DEFAULT_SYNC_METHOD.  Obviously xlogdefs.h needs to include <fcntl.h>
for itself to ensure stable results.