xapp_mkfontscale: enable compilation for the host and fix deps

mkfontscale on the host is required to build xfonts-encoding for the
target.

We also fix the dependencies. xlib_libX11 is not needed to compile
xapp_mkfontscale. However, xproto_xproto is needed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>