lib/c: add const qualifier to criu_set_service_binary

service_binary is either set to a const char * (CR_DEFAULT_SERVICE_BIN)
or to a user provided char *, but there is no reason to give a char *.
Users of such function will most likely provide a const char *,
that will generate a warning.
Thus, we add the const qualifier to better represent the usage of
service_binary, and avoid such warnings.

Signed-off-by: Ronny Chevalier <ronny.chevalier@hp.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>