powerpc: Fix compile errors with CONFIG_BUG=n

This makes sure we don't try to call find_bug or is_warning_bug when
CONFIG_BUG=n and CONFIG_XMON=y.  Otherwise we get these errors:

arch/powerpc/xmon/xmon.c: In function ‘print_bug_trap’:
arch/powerpc/xmon/xmon.c:1364: error: implicit declaration of function ‘find_bug’
arch/powerpc/xmon/xmon.c:1364: warning: assignment makes pointer from integer without a cast
arch/powerpc/xmon/xmon.c:1367: error: implicit declaration of function ‘is_warning_bug’
arch/powerpc/xmon/xmon.c:1374: error: dereferencing pointer to incomplete type
make[2]: *** [arch/powerpc/xmon/xmon.o] Error 1
make[1]: *** [arch/powerpc/xmon] Error 2
make: *** [sub-make] Error 2

Signed-off-by: Paul Mackerras <paulus@samba.org>