libxfont: make xproto a standard dependency
libxfont: make xproto a standard dependency

This patch makes xproto a standard dependency of libxfont, rather than a
build-time dependency
Building of X.Org font bottles fails with the following error message:

   checking for BDFTOPCF... configure: error: Package requirements (xfont) were not met:
   Package 'xproto', required by 'xfont', not found

LinuxbrewTestBot/homebrew-xorg,maxim-belkin/homebrew-xorg,Linuxbrew/homebrew-xorg