Add missing include for std::round

We had a compile failure on openSuse on build.kde.org. This is a try to
fix the failure with adding the include.

libinput/connection.cpp

@@ -38,6 +38,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <QThread>
 
 #include <libinput.h>
+#include <cmath>
 
 namespace KWin
 {