Include <errno.h> in waylandclipboard.cpp

Summary:
`errno.h` needs to be includes, as `errno`, `EAGAIN` and `EWOULDBLOCK` are used.

Without:
> --- helpers/xclipboardsync/CMakeFiles/org_kde_kwin_xclipboard_syncer.dir/all ---
> /wrkdirs/usr/ports/x11-wm/plasma5-kwin/work/kwin-5.7.95/helpers/xclipboardsync/waylandclipboard.cpp:110:25: error: use of undeclared identifier 'errno'
>         if (n == -1 && (errno == EAGAIN || errno == EWOULDBLOCK) && ++retryCount < 1000) {
>                         ^
> /wrkdirs/usr/ports/x11-wm/plasma5-kwin/work/kwin-5.7.95/helpers/xclipboardsync/waylandclipboard.cpp:110:34: error: use of undeclared identifier 'EAGAIN'
>         if (n == -1 && (errno == EAGAIN || errno == EWOULDBLOCK) && ++retryCount < 1000) {
>                                  ^
> /wrkdirs/usr/ports/x11-wm/plasma5-kwin/work/kwin-5.7.95/helpers/xclipboardsync/waylandclipboard.cpp:110:44: error: use of undeclared identifier 'errno'
>        if (n == -1 && (errno == EAGAIN || errno == EWOULDBLOCK) && ++retryCount < 1000) {
>                                           ^
> /wrkdirs/usr/ports/x11-wm/plasma5-kwin/work/kwin-5.7.95/helpers/xclipboardsync/waylandclipboard.cpp:110:53: error: use of undeclared identifier 'EWOULDBLOCK'
>        if (n == -1 && (errno == EAGAIN || errno == EWOULDBLOCK) && ++retryCount < 1000) {
>                                                    ^
> 4 errors generated.
> *** [helpers/xclipboardsync/CMakeFiles/org_kde_kwin_xclipboard_syncer.dir/waylandclipboard.cpp.o] Error code 1

Reviewers: graesslin

Reviewed By: graesslin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D2794

helpers/xclipboardsync/waylandclipboard.cpp

@@ -35,6 +35,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <QMimeType>
 #include <qplatformdefs.h>
 
+#include <errno.h>
 #include <unistd.h>
 
 using namespace KWayland::Client;