Include what you use in HexagonMCInstrInfo.cpp

Include what you use in HexagonMCInstrInfo.cpp

HexagonMCInstrInfo.cpp was not using any APIs from Hexagon.h.  Doing so
is problematic from include-what-you-use perspective, but it is also a
layering issue (it creates a dependency cycle between the primary
Hexagon target library and the MCTargetDesc library).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@362387 91177308-0d34-0410-b5e6-96231b3b80d8