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/xkb_unicode.c

@@ -29,6 +29,7 @@
 
 #include "internal.h"
 
+#if defined(_GLFW_X11) || defined(_GLFW_WAYLAND)
 
 /*
  * Marcus: This code was originally written by Markus G. Kuhn.
@@ -940,3 +941,5 @@ uint32_t _glfwKeySym2Unicode(unsigned int keysym)
     return GLFW_INVALID_CODEPOINT;
 }
 
+#endif // _GLFW_WAYLAND or _GLFW_X11
+