Added override specifier to MemCheck plugin.
Added override specifier to MemCheck plugin.

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