pgsql: Suppress warning when compiling with -DPROFILE_PID_DIR:

Log Message:
-----------
Suppress warning when compiling with -DPROFILE_PID_DIR: sys/stat.h is
supposed to be included when using mkdir().

Modified Files:
--------------
pgsql/src/backend/storage/ipc:
ipc.c (r1.96 -> r1.97)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/ipc/ipc.c.diff?r1=1.96&r2=1.97)