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