Use find_program instead of pkg_get_variable for wayland-scanner

wayland-scanner executable should be located for the host system.

When wayland-scanner.pc also present in the target sysroot pkg_get_variable() will incorrectly return executable path for the target platform.

System and host sysroot executables should be located with find_program() to avoid such issue.

Signed-off-by: Daniel Levin daniel.levin@amd.com