package/cmocka: disable documentation building

package/cmocka: disable documentation building

cmocka tries to find the Doxygen binary by default and if successful,
it tries to download style sheets and breaks with the following message:

CMake Error at
doxygen-awesome-css-subbuild/doxygen-awesome-css-populate-prefix/src/doxygen-awesome-css-populate-stamp/download-doxygen-awesome-css-populate.cmake:163
(message):
  Each download failed!

    error: downloading
'https://github.com/jothepro/doxygen-awesome-css/archive/refs/tags/v2.4.1.tar.gz'
failed
          status_code: 1
          status_string: "Unsupported protocol"
          log:
          --- LOG BEGIN ---
          Protocol "https" not supported

closing connection #-1

Disable the Doxygen package search entirely.

Fixes:
https://autobuild.buildroot.org/results/e6a04cee8bc3028bd8a1a535c2852e60f362c4ba/

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>