meson: don't require xf86bigfontproto if not building xf86bigfont support

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
3 jobs for master in 6 minutes and 27 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #591152
debian-testing

00:01:13

 
  Build And Test
passed #591153
autotools-build-and-test

00:05:14

passed #591154
meson-build-and-test

00:01:52