Explicitly disable inclusion for test and examples

Thank you, Travis CI, for reminding me that one cannot disable a header
with inclusion guards if it doesn't exist.

examples/sharing.c

@@ -24,6 +24,7 @@
 //========================================================================
 
 #include <glad/gl.h>
+#define GLFW_INCLUDE_NONE
 #include <GLFW/glfw3.h>
 
 #include <stdio.h>