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/io/collada/BCAnimationSampler.cpp

@@ -27,20 +27,21 @@
 #include "ExportSettings.h"
 #include "collada_utils.h"
 
-extern "C" {
 #include "BKE_action.h"
 #include "BKE_constraint.h"
 #include "BKE_key.h"
 #include "BKE_lib_id.h"
 #include "BKE_main.h"
 #include "BKE_material.h"
+
 #include "BLI_listbase.h"
+
 #include "DNA_anim_types.h"
 #include "DNA_constraint_types.h"
 #include "DNA_key_types.h"
 #include "DNA_scene_types.h"
+
 #include "ED_object.h"
-}
 
 static std::string EMPTY_STRING;
 static BCAnimationCurveMap BCEmptyAnimationCurves;