gdb: constify remote files_info

gdb: constify remote files_info

The sim and m32r remote targets declare a local "file" variable and only
assign const strings to it before passing it to a printf() func.  So add
const markings to avoid gcc warnings like:
gdb/remote-sim.c: In function 'gdbsim_files_info':
gdb/remote-sim.c:789: warning: initialization discards qualifiers
	from pointer target type

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