libxcb: re-disable building documentation for the host

Fixes http://autobuild.buildroot.net/results/98f/98f8dd2d1a9f8fac3024caf5aa0ca54fcf320389/



Commit 862e1221 (Bump xcb-proto & libxcb version to 1.10) accidently
dropped the --disable-build-docs configure arguments for the host variant,
which in the best case slows down the build for no use, and on certain
hosts breaks the build as doxygen runs out of memory.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>