trace: Add a const version of the "arg" method
trace: Add a const version of the "arg" method

With this the class trace::Call can be used const-correct

Signed-off-by: Gert Wollny <a4abb7b2c48b4038af2f307a789969b4cce91ae7@collabora.com>

apitrace/apitrace,apitrace/apitrace,apitrace/apitrace,apitrace/apitrace