Cycles: Remove Compilation Warning

ROCm 3.9 already defined `NULL`. This patch will first check if it was
already defined to remove compilation warnings.

NOTE: This doesn't add official support for ROCm as it still fails to
render correctly (crashes with default cube).

Reviewed By: Brecht van Lommel

Differential Revision: https://developer.blender.org/D9610

intern/cycles/kernel/kernel_compat_opencl.h

@@ -162,7 +162,9 @@
 #define kernel_tex_fetch(tex, index) kernel_tex_array(tex)[(index)]
 
 /* define NULL */
-#define NULL 0
+#ifndef NULL
+#  define NULL ((void *)0)
+#endif
 
 /* enable extensions */
 #ifdef __KERNEL_CL_KHR_FP16__