powerpc: Export flush_icache_range

Commit aac416fc (lkdtm: flush icache and report actions) calls
flush_icache_range from a module. It's exported on most architectures
that implement it, but not on powerpc. This patch exports it to fix
the module link failure.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>