add c++11 override marker to overridden methods

C++11 adds the 'override' specifier to mark that a method is overriding
a base class method and error out if not. We hide it in the APT_OVERRIDE
macro to ensure that we keep compiling in pre-c++11 standards.

Reported-By: clang-modernize -add-override -override-macros
Git-Dch: Ignore