This is needed when packages lookup the mpv library via pkg-config, such as
done by plasmatube.
Prior to this change, the following would fail (this is what CMake's
'pkg_check_modules' does):
$ guix shell --pure pkg-config mpv \
-- pkg-config --print-errors --short-errors --exists mpv
Package 'wayland-client', required by 'mpv', not found
* gnu/packages/video.scm (mpv)
[inputs]: Move alsa-lib, ffmpeg, jack-1, libass, libbluray, libcaca,
libcdio-paranoia, libdvdnav, libjpeg-turbo, libplacebo, libva, libvdpau,
libx11, libxext, libxinerama, libxpresent, libxrandr, libxscrnsaver, lua-5.2,
mesa, pulseaudio, shaderc, wayland, wayland-protocols, zimg and zlib to...
[propagated-inputs]: ... here.
Change-Id: Ia0cd4e33a8b4df4b060546c979d00be0b2b3977e
f3fdb4e041