user/gst-plugins-base: add cmd:which for orcc, not itself

Having gst-plugins-base installed while building a newer version
causes a build failure.

user/gst-plugins-base/APKBUILD

@@ -13,7 +13,7 @@ replaces="gst-plugins-base1"
 makedepends="alsa-lib-dev cdparanoia-dev expat-dev glib-dev
 	gobject-introspection-dev gstreamer-dev libice-dev libogg-dev libsm-dev
 	libtheora-dev libvorbis-dev libx11-dev libxt-dev libxv-dev mesa-dev
-	opus-dev orc-dev pango-dev perl"
+	opus-dev orc-dev pango-dev perl cmd:which !gst-plugins-base"
 checkdepends="orc-compiler"
 source="https://gstreamer.freedesktop.org/src/gst-plugins-base/gst-plugins-base-$pkgver.tar.xz
 	endian.patch