powerpc: Do not ignore arch/powerpc/include

Back when .gitignore file was added to arch/powerpc/ in 06f2138e ([POWERPC]
Add files build to .gitignore, 2006-11-26), there indeed was nothing
tracked in the ignored hierarchy and ignoring everything made sense.  But
we have very many tracked files there these days, and having a higher
level .gitignore that ignores everything is asking for future troubles..

This should have been part of b8b572e1 (powerpc: Move include files to
arch/powerpc/include/asm, 2008-08-01).

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>