Include what you use in HexagonShuffler.cpp

Include what you use in HexagonShuffler.cpp

HexagonShuffler.cpp was not using any APIs from Hexagon.h, and was only
including it for transitive dependencies.  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@362384 91177308-0d34-0410-b5e6-96231b3b80d8