sim: add const markings to env string

sim: add const markings to env string

The sim-options code assigns a const string to "char *" in decoding the
current environment settings, and only uses it to pass to a printf.  GCC
outputs the warnings:
common/sim-options.c: In function 'standard_option_handler':
common/sim-options.c:271: warning: assignment discards qualifiers from pointer
target type
common/sim-options.c:272: warning: assignment discards qualifiers from pointer
target type
common/sim-options.c:273: warning: assignment discards qualifiers from pointer
target type

So I've committed this as "obvious".

Signed-off-by: Mike Frysinger <vapier@gentoo.org>