Cleanup: correct usage of extern-C blocks in various places

This removes extern-C blocks around other includes and adds
such blocks for some headers that need them.

source/blender/python/intern/bpy_traceback.h

@@ -21,6 +21,14 @@
 #ifndef __BPY_TRACEBACK_H__
 #define __BPY_TRACEBACK_H__
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 void python_script_error_jump(const char *filepath, int *lineno, int *offset);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* __BPY_TRACEBACK_H__ */