pty: include linux/ioctl.h for TIOCSIG

TIOCSIG is linux specific, so include the linux ioctl header to make sure
it's defined. We currently rely on some rather non-obvious recursive
includes. Make sure its always defined regardless of the system headers.

src/shared/pty.c

@@ -45,6 +45,7 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <limits.h>
+#include <linux/ioctl.h>
 #include <pty.h>
 #include <signal.h>
 #include <stdbool.h>