[NFC] Add missing include to fix modules build
[NFC] Add missing include to fix modules build

This header doesn't seem to be parsable on its own and breaks the module build therefore with
the following error:

While building module 'LLVM_Backend' imported from llvm-project/llvm/lib/CodeGen/MachineScheduler.cpp:14:
In file included from <module-includes>:62:
llvm-project/llvm/include/llvm/CodeGen/MachinePipeliner.h:91:20: error: declaration of 'AAResultsWrapperPass' must be imported from module 'LLVM_Analysis.AliasAnalysis' before it is required
    AU.addRequired<AAResultsWrapperPass>();
                   ^
llvm-project/llvm/include/llvm/Analysis/AliasAnalysis.h:1157:7: note: previous declaration is here
class AAResultsWrapperPass : public FunctionPass {
      ^
llvm-project/llvm/lib/CodeGen/MachineScheduler.cpp:14:10: fatal error: could not build module 'LLVM_Backend'
 ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@375433 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm