missing.h: add BTN_DPAD_UP

As explained in issue 6267 of systemd [1], Linux < 3.11 does not
provide definitions for BTN_DPAD_{UP,RIGHT}, which were introduced in
[2].

This patch fixes this issue.

[1] systemd/systemd#6267
[2] torvalds/linux@9ee2748

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>

src/shared/missing.h

@@ -183,3 +183,8 @@ static inline int name_to_handle_at(int fd, const char *name, struct file_handle
 #ifndef KEY_ALS_TOGGLE
 #define KEY_ALS_TOGGLE 0x7a
 #endif
+
+#ifndef BTN_DPAD_UP
+#define BTN_DPAD_UP 0x220
+#define BTN_DPAD_RIGHT 0x223
+#endif