Include <errno.h> to fix build on FreeBSD

Summary:
/wrkdirs/usr/ports/x11/kf5-kwayland/work/kwayland-5.28.0/src/client/plasmawindowmanagement.cpp:489:25: error: use of undeclared identifier 'errno'

if (n == -1 && (errno == EAGAIN) && ++retryCount < 1000) {
                ^

/wrkdirs/usr/ports/x11/kf5-kwayland/work/kwayland-5.28.0/src/client/plasmawindowmanagement.cpp:489:34: error: use of undeclared identifier 'EAGAIN'

if (n == -1 && (errno == EAGAIN) && ++retryCount < 1000) {
                         ^

Reviewers: graesslin, bshah

Reviewed By: bshah

Subscribers: plasma-devel

Tags: Plasma on Wayland

Differential Revision: https://phabricator.kde.org/D3289