x11-wm/plasma5-kwin: add missing errno.h include

This fixes the build on FreeBSD 12

Reported by:	VVD

x11-wm/plasma5-kwin/files/patch-src_backends_drm_drm__pipeline__legacy.cpp

+--- src/backends/drm/drm_pipeline_legacy.cpp.orig	2022-02-09 06:05:36 UTC
++++ src/backends/drm/drm_pipeline_legacy.cpp
+@@ -14,6 +14,8 @@
+ #include "drm_object_connector.h"
+ #include "logging.h"
+ 
++#include <errno.h>
++
+ namespace KWin
+ {
+