Refactor: Remove extern "C" from most headers

The only remaining code in source/blender that must be compiled as C
is now datatoc generated code and the DNA defaults that use designated
initializers.

Pull Request: https://projects.blender.org/blender/blender/pulls/134469

source/blender/freestyle/intern/python/BPy_UnaryPredicate1D.cpp

@@ -22,10 +22,6 @@
 #include "UnaryPredicate1D/BPy_TrueUP1D.h"
 #include "UnaryPredicate1D/BPy_WithinImageBoundaryUP1D.h"
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 using namespace Freestyle;
 
 ///////////////////////////////////////////////////////////////////////////////////////////
@@ -246,7 +242,3 @@ PyTypeObject UnaryPredicate1D_Type = {
 };
 
 ///////////////////////////////////////////////////////////////////////////////////////////
-
-#ifdef __cplusplus
-}
-#endif