Avoid dead code warning when using a constant boolean

Avoid dead code warning when using a constant boolean

The value of `sign_bit` is platform-dependent but constant at compile
time. Use a cast to convert the boolean `sign_bit` to 0 or 1 instead of
ternary `?:` because one branch of the conditional is dead code. (We
could leave out the cast to `size_t` but our style prefers to handle
booleans more explicitly, hence the `?:` that caused the issue.)

    *** CID 358310:  Possible Control flow issues  (DEADCODE)
    /lib/isc/resource.c: 118 in isc_resource_setlimit()
    112     		 * rlim_t, and whether rlim_t has a sign bit.
    113     		 */
    114     		isc_resourcevalue_t rlim_max = UINT64_MAX;
    115     		size_t wider = sizeof(rlim_max) - sizeof(rlim_t);
    116     		bool sign_bit = (double)(rlim_t)-1 < 0;
    117
    >>>     CID 358310:  Possible Control flow issues  (DEADCODE)
    >>>     Execution cannot reach the expression "1" inside this statement: "rlim_max >>= 8UL * wider + ...".
    118     		rlim_max >>= CHAR_BIT * wider + (sign_bit ? 1 : 0);
    119     		rlim_value = ISC_MIN(value, rlim_max);
    120     	}
    121
    122     	rl.rlim_cur = rl.rlim_max = rlim_value;
    123     	unixresult = setrlimit(unixresource, &rl);