Linux: Fix joystick without buttons being ignored

Joysticks in Linux without buttons were ignored by device enumeration.
Remove the mandatory button attribute from detection.

Fixes glfw#2042
Closes glfw#2043

src/linux_joystick.c

@@ -157,7 +157,7 @@ static GLFWbool openJoystickDevice(const char* path)
     }
 
     // Ensure this device supports the events expected of a joystick
-    if (!isBitSet(EV_KEY, evBits) || !isBitSet(EV_ABS, evBits))
+    if (!isBitSet(EV_ABS, evBits))
     {
         close(linjs.fd);
         return GLFW_FALSE;