bdftopcf: require xproto and fontsproto at build time (#6112)

These are technically "link" depends of libxfont, but they have no
actual libraries.

Fixes #5936.

var/spack/repos/builtin/packages/bdftopcf/package.py

@@ -41,4 +41,6 @@ class Bdftopcf(AutotoolsPackage):
     depends_on('libxfont')
 
     depends_on('pkg-config@0.9.0:', type='build')
+    depends_on('xproto', type='build')
+    depends_on('fontsproto', type='build')
     depends_on('util-macros', type='build')