Fix relative include path

This is one of those things I need to figure out one of these days. With
a CMake build system, you can put pretty much anything in between
quotes, and it'll find it even if the relative path is wrong. Then with
a makefile, they have to be mostly correct, and then finally on more
obscure platforms, the build blows up if they are even slightly
incorrect.
Huh!

src/utils/loaders/sceneloader.c

@@ -22,7 +22,7 @@
 #include "../logging.h"
 #include "../fileio.h"
 #include "../string.h"
-#include "vendored/cJSON.h"
+#include "../../vendored/cJSON.h"
 #ifdef WINDOWS // Sigh...
 #include <malloc.h>
 #else