sim: constify save_data()

sim: constify save_data()

The local save_data() function takes a pointer to a buffer and only uses it as
the source to the memcpy() function.  Since it is given const strings, GCC
likes to spit out warnings:
common/sim-trace.c: In function 'trace_prefix':
common/sim-trace.c:697: warning: passing argument 5 of 'save_data' discards
	qualifiers from pointer target type

So I've committed this as obvious.

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