test/bench/distribution.h: pass rngen_t by reference

CID 1135927 (#1 of 1): Big parameter passed by value (PASS_BY_VALUE)
 pass_by_value: Passing parameter rng of type rngen_t (size 1416 bytes)
 by value.

CID 1135928 (#1 of 1): Big parameter passed by value (PASS_BY_VALUE)
 pass_by_value: Passing parameter rng of type rngen_t (size 1416 bytes)
 by value.

CID 1135929 (#1 of 1): Big parameter passed by value (PASS_BY_VALUE)
 pass_by_value: Passing parameter rng of type rngen_t (size 1416 bytes)
 by value.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>