Fixed build with older Linux kernel headers
diff --git a/src/joystick/linux/SDL_sysjoystick.c b/src/joystick/linux/SDL_sysjoystick.c
index a5e80d6..9172165 100644
--- a/src/joystick/linux/SDL_sysjoystick.c
+++ b/src/joystick/linux/SDL_sysjoystick.c
@@ -56,6 +56,12 @@
#ifndef SYN_DROPPED
#define SYN_DROPPED 3
#endif
+#ifndef BTN_NORTH
+#define BTN_NORTH 0x133
+#endif
+#ifndef BTN_WEST
+#define BTN_WEST 0x134
+#endif
#ifndef BTN_DPAD_UP
#define BTN_DPAD_UP 0x220
#endif