gitignore: version headers are in libweston/

Switching back and forth to e.g. weston 1.11 branch can leave
these files in src/. Fix the gitignores to not hide them from
git-status.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

.gitignore

@@ -89,8 +89,6 @@ weston-tablet-shell
 weston-terminal
 weston-multi-resource
 weston-simple-im
-git-version.h
-version.h
 weston
 weston-launch
 spring-tool
@@ -105,6 +103,9 @@ weston.1
 weston-drm.7
 weston.ini.5
 
+/libweston/git-version.h
+/libweston/version.h
+
 /tests/weston-ivi.ini
 internal-screenshot-00.png