add !meta-poky to .gitignore file

Another meta-yocto -> meta-poky transition item. Git complained
on commit that files in meta-poky could not be committed.

Signed-off-by: Bill Randle <william.c.randle@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

.gitignore

@@ -14,6 +14,7 @@ hob-image-*.bb
 *.orig
 *.rej
 *~
+!meta-poky
 !meta-yocto
 !meta-yocto-bsp
 !meta-yocto-imported