Include what you use in HexagonELFObjectWriter.cpp

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