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).

llvm-svn: 362384