powerpc/lib: Declare static methods

Functions do_stf_{entry,exit}_barrier_fixups are static but not declared as
such. This was detected by `sparse` tool with the following warning:

	arch/powerpc/lib/feature-fixups.c:121:6: warning: symbol 'do_stf_entry_barrier_fixups' was not declared. Should it be static?

	arch/powerpc/lib/feature-fixups.c:171:6: warning: symbol 'do_stf_exit_barrier_fixups' was not declared. Should it be static?

This patch declares both functions as static, as they are only called by
do_stf_barrier_fixups(), which is in the same source code file.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>