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/cocoa_joystick.m

@@ -29,6 +29,8 @@
 
 #include "internal.h"
 
+#if defined(_GLFW_COCOA)
+
 #include <unistd.h>
 #include <ctype.h>
 #include <string.h>
@@ -476,3 +478,5 @@ void _glfwUpdateGamepadGUIDCocoa(char* guid)
     }
 }
 
+#endif // _GLFW_COCOA
+