package/wayland-protocols: fix build without host-wayland

package/wayland-protocols: fix build without host-wayland

Don't require wayland-scanner if tests are disabled to avoid the
following build failure raised since bump to version 1.23 in commit
7eedc9cc1e0c3cce1c18cb150157cb2346075b23:

Build-time dependency wayland-scanner found: NO (tried pkgconfig and cmake)

../output-1/build/wayland-protocols-1.23/meson.build:11:0: ERROR: Dependency "wayland-scanner" not found, tried pkgconfig and cmake

Fixes:
 - http://autobuild.buildroot.org/results/2744e50465a9cd9d3726d23298ad6c943ef49a21

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>