Cleanup: reduce indirect DNA header inclusion

Remove DNA headers, using forward declarations where possible.

Also removed duplicate header, header including it's self
and unnecessary inclusion of libc system headers from BKE header.

source/blender/python/generic/idprop_py_api.c

@@ -28,6 +28,8 @@
 
 #include "BKE_idprop.h"
 
+#include "DNA_ID.h" /* ID property definitions. */
+
 #define USE_STRING_COERCE
 
 #ifdef USE_STRING_COERCE