[GDB, Hurd] Fix build; 'target_waitstatus_to_string' call

Recent commit c29705b7 removed an incomplete
local implementation in favor of 'target_waitstatus_to_string' (thanks!), but
introduced a small typing error:

    In file included from [...]/gdb/gnu-nat.c:24:0:
    [...]/gdb/gnu-nat.c: In member function 'virtual ptid_t gnu_nat_target::wait(ptid_t, target_waitstatus*, int)':
    [...]/gdb/gnu-nat.c:1652:43: error: cannot convert 'target_waitstatus**' to 'const target_waitstatus*' for argument '1' to 'std::__cxx11::string target_waitstatus_to_string(const target_waitstatus*)'
           target_waitstatus_to_string (&status).c_str ());
                                               ^
    [...]/gdb/gnu-nat.h:119:32: note: in definition of macro 'debug'
            __FILE__ , __LINE__ , ##args); } while (0)
                                    ^~~~
    [...]/gdb/gnu-nat.c:1650:3: note: in expansion of macro 'inf_debug'
       inf_debug (inf, "returning ptid = %s, %s",
       ^~~~~~~~~

	gdb/
	* gnu-nat.c (gnu_nat_target::wait): Fix
	target_waitstatus_to_string call.