Add conditional compilation for platform units

This is a step towards being able to compile GLFW manually without
needing to duplicate a lot of platform- or OS-specific logic.

src/linux_joystick.c

@@ -29,6 +29,8 @@
 
 #include "internal.h"
 
+#if defined(GLFW_BUILD_LINUX_JOYSTICK)
+
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/inotify.h>
@@ -429,3 +431,5 @@ void _glfwUpdateGamepadGUIDLinux(char* guid)
 {
 }
 
+#endif // GLFW_BUILD_LINUX_JOYSTICK
+