[NFC] Add missing bit.h to SubtargetFeature.h

SubtargetFeature.h uses llvm/ADT/bit.h.

Reviewed By: compnerd, mib

Differential Revision: https://reviews.llvm.org/D154664

Author: etcwilde

llvm/include/llvm/TargetParser/SubtargetFeature.h

@@ -20,6 +20,7 @@
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/bit.h"
 #include "llvm/Support/MathExtras.h"
 #include <array>
 #include <initializer_list>