Fix: Compiler warning about format string

Don't pass string class into printf("%s"), only char* allowed.

intern/cycles/bvh/embree.cpp

@@ -178,7 +178,7 @@ void BVHEmbree::build(Progress &progress,
   rtcCommitScene(scene);
 }
 
-string BVHEmbree::get_last_error_message()
+const char *BVHEmbree::get_last_error_message()
 {
   const RTCError error_code = rtcGetDeviceError(rtc_device);
   switch (error_code) {

intern/cycles/bvh/embree.h

@@ -41,7 +41,7 @@ class BVHEmbree : public BVH {
   bool offload_scenes_to_gpu(const vector<RTCScene> &scenes);
 #  endif
 
-  string get_last_error_message();
+  const char *get_last_error_message();
 
   RTCScene scene;