sim: tweak load buffer type to avoid signed warnings

sim: tweak load buffer type to avoid signed warnings

The sim_load_file func creates a buffer with arbitrary data in it (reads
it via the bfd).  It then passes it on to a sim_write_fn which expects a
unsigned char buffer.  Since sim_load_file itself doesn't care about the
contents, tweak the type to avoid signed mismatch warnings from gcc:

common/sim-load.c: In function ‘sim_load_file’:
common/sim-load.c:143: warning: pointer targets in passing argument 3 of ‘do_write’ differ in signedness
common/sim-load.c:143: note: expected ‘const unsigned char *’ but argument is of type ‘char *’

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