guile: Simplify ioscm_make_gdb_stdio_port

guile: Simplify ioscm_make_gdb_stdio_port

As pointed out by Pedro, it's clearer to do it this way.  We can trust
that scm_mode_bits won't try to modify our string, even though it takes
a non-const char *.

gdb/ChangeLog:

	* guile/scm-ports.c (ioscm_make_gdb_stdio_port): Do not pass a
	local char array to scm_mode_bits, use a cast instead.