[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.

llvm-svn: 375433

Author: Teemperor

llvm/include/llvm/CodeGen/MachinePipeliner.h

@@ -40,6 +40,8 @@
 #ifndef LLVM_LIB_CODEGEN_MACHINEPIPELINER_H
 #define LLVM_LIB_CODEGEN_MACHINEPIPELINER_H
 
+#include "llvm/Analysis/AliasAnalysis.h"
+
 #include "llvm/CodeGen/MachineDominators.h"
 #include "llvm/CodeGen/RegisterClassInfo.h"
 #include "llvm/CodeGen/ScheduleDAGInstrs.h"