Suppress compiler warnings about inconsistent override.

Suppress compiler warnings about inconsistent override.

The introduction of the 'override' keyword in GiNaC in d5b86dd10 was
deliberately incomplete: Duplication of code in macros was avoided.
It turns out that some compilers (e.g. g++ -Wsuggest-override or clang++
-Wall) complain if the 'override' keyword is used inconsistently. Let's
make this consistent now, even if it leads to the introduction of two
more macros.