[NFC] #include MCSymbol.h to avoid an incomplete type

Note: this header already has a dep on MC headers via MCContext.

llvm/include/llvm/CodeGen/MachineModuleInfo.h

@@ -34,6 +34,7 @@
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/IR/PassManager.h"
 #include "llvm/MC/MCContext.h"
+#include "llvm/MC/MCSymbol.h"
 #include "llvm/Pass.h"
 #include <memory>
 #include <utility>
@@ -45,7 +46,6 @@ class Function;
 class LLVMTargetMachine;
 class MachineFunction;
 class Module;
-class MCSymbol;
 
 //===----------------------------------------------------------------------===//
 /// This class can be derived from and used by targets to hold private