[POWERPC] Only ignore arch/ppc/include, not arch/ppc/boot/include

arch/ppc/.gitignore shouldn't exclude arch/ppc/boot/include, so
this makes arch/ppc/.gitignore more specific.
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/ppc/.gitignore

-include
+/include