Cleanup: take includes out of 'extern "C"' blocks

Surrounding includes with an 'extern "C"' block is not necessary anymore.
Also that made it harder to add any C++ code to some headers, or include headers
that have "optional" C++ code like `MEM_guardedalloc.h`.

I tested compilation on linux and windows (and got help from @LazyDodo).
If this still breaks compilation due to some linker error, the header containing
the symbol in question is probably missing an 'extern "C"' block.

Differential Revision: https://developer.blender.org/D7653

source/blender/python/BPY_extern.h

@@ -35,12 +35,12 @@ struct bContext;
 struct bContextDataResult;
 struct bPythonConstraint; /* DNA_constraint_types.h */
 
+#include "BLI_utildefines.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#include "BLI_utildefines.h"
-
 void BPY_pyconstraint_exec(struct bPythonConstraint *con,
                            struct bConstraintOb *cob,
                            struct ListBase *targets);