Cleanup: Remove using-directive from freestyle headers

The header files in freestyle utilize the using-directive at the global
file scope. This is a bad practice as it pollutes the global name space
causing possible ambiguous reference compilation errors. In particular,
the DNA files that are included by freestyle will cause those ambiguous
reference errors when the developers adds a DNA member with a type name
that also exist in the Freestyle name space, such as Curve and possibly
others.

This patch does the minimal work needed to resolve that by moving the
using-directives from the headers into the corresponding translation
units.

Reviewed By: Brecht

Differential Revision: https://developer.blender.org/D10351

source/blender/freestyle/intern/python/BPy_Interface1D.cpp

@@ -35,6 +35,8 @@
 extern "C" {
 #endif
 
+using namespace Freestyle;
+
 ///////////////////////////////////////////////////////////////////////////////////////////
 
 //-------------------MODULE INITIALIZATION--------------------------------