Include what you use in HexagonELFObjectWriter.cpp

HexagonELFObjectWriter.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: 362376