Resolve "Voxel cone tracing"

Closes #82 (closed)

projects/voxelization/resources/shaders/brdf.inc

+#ifndef BRDF_INC
+#define BRDF_INC
+
 const float pi = 3.1415; 
 
 vec3 lambertBRDF(vec3 albedo){
@@ -23,4 +26,6 @@ float GGXSmithShadowingPart(float r, float cosTheta){
 
 float GGXSmithShadowing(float r, float NoV, float NoL){
     return GGXSmithShadowingPart(r, NoV) * GGXSmithShadowingPart(r, NoL);
-}
\ No newline at end of file
+}
+
+#endif // #ifndef BRDF_INC
\ No newline at end of file