libwebp: fix wrong DEPENDS

* there is no such thing as packagespecific build-time DEPENDS
* RDEPENDS_${PN}-gif2web isn't needed, because it's detected
  automatically by shlibs providers code
* fixes following floating dependencies:
  WARN: libwebp: libwebp-gif2webp rdepends on giflib but it isn't a build dependency?

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>