pgsql: Fix a subtle bug exposed by recent wal_sync_method

Log Message:
-----------
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.

Modified Files:
--------------
pgsql/src/backend/access/transam:
xlog.c (r1.310 -> r1.311)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.310&r2=1.311)
pgsql/src/include/access:
xlogdefs.h (r1.21 -> r1.22)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlogdefs.h?r1=1.21&r2=1.22)