xwayland: fix missing include of <errno.h>

It's much cleaner to always include directly what one needs,
instead of relying on very indirect including.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1435>

hw/xwayland/xwayland-xtest.c

@@ -29,6 +29,7 @@
 #include <inpututils.h>
 #include <libgen.h>
 #include <unistd.h>
+#include <errno.h>
 
 #include <libei.h>