Merge pull request #255 from DougGregor/include-generic-signature-stable

Add missing #include for Swift's GenericSignature.

Author: DougGregor

source/Symbol/SwiftASTContext.cpp

@@ -25,6 +25,7 @@
 #include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/IRGenOptions.h"
 #include "swift/AST/ASTMangler.h"
+#include "swift/AST/GenericSignature.h"
 #include "swift/AST/NameLookup.h"
 #include "swift/AST/SearchPathOptions.h"
 #include "swift/AST/Type.h"