parisc: asm: psw.h: missing header guard

The psw.h header file contains #ifndef directive of the guard,
but the complimentary #define directive is missing. The patch
adds the appropriate #define to fix the header guard.

Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Helge Deller <deller@gmx.de>