Cleanup: Move BKE_material.h to C++

The `BKE_material.h` is only renamed to `.hh` to preserve
the history of the file. Changes to the file are done in
the following commit.

source/blender/freestyle/intern/python/BPy_Freestyle.cpp

@@ -44,6 +44,7 @@
 
 #include "BKE_colorband.hh"  /* BKE_colorband_evaluate() */
 #include "BKE_colortools.hh" /* BKE_curvemapping_evaluateF() */
+#include "BKE_material.hh"   /* ramp_blend() */
 
 #ifdef __cplusplus
 extern "C" {
@@ -138,8 +139,6 @@ static int ramp_blend_type(const char *type)
   return -1;
 }
 
-#include "BKE_material.h" /* ramp_blend() */
-
 PyDoc_STRVAR(
     /* Wrap. */
     Freestyle_blendRamp___doc__,