build: print missing dependency error to stderr

Allow to see the actual error even when compiling with V=w/V=1 only.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37388 3c298f89-4303-0410-b956-a3cf2f4a3e73