[POWERPC] Fix arch/powerpc/platforms/powermac/pic.c when !CONFIG_ADB_PMU

When building arch/powerpc/platforms/powermac/pic.c when !CONFIG_ADB_PMU
we get the following warnings:

arch/powerpc/platforms/powermac/pic.c: In function 'pmacpic_find_viaint':
arch/powerpc/platforms/powermac/pic.c:623: warning: label 'not_found' defined but not used

This fixes it.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>