Removed unused variable warnings from mul_hi builtins.
Removed unused variable warnings from mul_hi builtins.

bitfusionio/Oclgrind,mpflanzer/Oclgrind,surround-io/oclgrind,mpflanzer/Oclgrind,mpflanzer/Oclgrind,bitfusionio/Oclgrind,mpflanzer/Oclgrind,bitfusionio/Oclgrind,bitfusionio/Oclgrind,surround-io/oclgrind,surround-io/oclgrind,surround-io/oclgrind