xfree86: int10: 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/xfree86/int10/generic.c

@@ -7,6 +7,7 @@
 #include <xorg-config.h>
 #endif
 
+#include <errno.h>
 #include <string.h>
 #include <unistd.h>