Fixed build on Linux(Ubuntu).

Contributed by @shahid-pk.

Closes pull request #360.

build/Tests.lua

@@ -49,6 +49,7 @@ function SetupTestGeneratorProject(name)
 
     links
     {
+      "System.Core",
       "CppSharp.AST",
       "CppSharp.Generator",
       "CppSharp.Generator.Tests"

src/CppParser/AST.cpp

@@ -591,7 +591,7 @@ DEF_STRING(TranslationUnit, FileName)
 DEF_VECTOR(TranslationUnit, MacroDefinition*, Macros)
 
 NativeLibrary::NativeLibrary()
-    : ArchType(ArchType::UnknownArch)
+    : ArchType(AST::ArchType::UnknownArch)
 {
 }