CMake: mark package variables as advanced

Silence pkg_check_modules message.

build/cmake/modules/FindGTK3.cmake

@@ -29,7 +29,7 @@
 # OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
 # ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 find_package(PkgConfig)
-pkg_check_modules(GTK3 gtk+-3.0)
+pkg_check_modules(GTK3 QUIET gtk+-3.0)
 set(VERSION_OK TRUE)
 if (GTK3_VERSION)
 if (GTK3_FIND_VERSION_EXACT)