sim_core.c: fix implimentation of ror instruction

prior implimentation just cleared n flag while specification states n should be set or cleared according to the most significant bit of the result.

bug find credit goes to: Shay Green <gblargg@gmail.com>

	modified:   simavr/sim/sim_core.c
Merge pull request #29 from bsekisser/sim-core-ror-bugfix

sim_core.c: fix implimentation of ror instruction