[LLVM-C] Fix name mangling on AggressiveInstCombine

Similarly to rL336736, at least one more C API function does not
properly get declared as extern "C" due to a missing header, causing
name mangling and linking errors.

This patch fixes calls to LLVMAddAggressiveInstCombinerPass().

Differential Revision: https://reviews.llvm.org/D49416

Reviewed By: whitequark

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337264 91177308-0d34-0410-b5e6-96231b3b80d8

Author: whitequark

lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

@@ -16,6 +16,7 @@
 #include "llvm/Transforms/AggressiveInstCombine/AggressiveInstCombine.h"
 #include "AggressiveInstCombineInternal.h"
 #include "llvm-c/Initialization.h"
+#include "llvm-c/Transforms/Scalar.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/BasicAliasAnalysis.h"
 #include "llvm/Analysis/GlobalsModRef.h"