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/compositor/operations/COM_VariableSizeBokehBlurOperation.cpp

@@ -20,9 +20,7 @@
 #include "BLI_math.h"
 #include "COM_OpenCLDevice.h"
 
-extern "C" {
 #include "RE_pipeline.h"
-}
 
 VariableSizeBokehBlurOperation::VariableSizeBokehBlurOperation() : NodeOperation()
 {