[gdb, hurd] Address "ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]" diagnostics

... that appeared with 9bf2a700667c53003ece783c05e8b355801105f2
"-Wwrite-strings: Remove -Wno-write-strings".

	gdb/
	* gnu-nat.c (gnu_write_inferior, parse_int_arg, _parse_bool_arg)
	(check_empty): Use "const char *".
2 files changed