Teach cppcheck that fatal() does not return

Teach cppcheck that fatal() does not return

cppcheck is not aware that the bin/dnssec/dnssectool.c:fatal() function
does not return.  This triggers certain cppcheck 2.2 false positives,
for example:

    bin/dnssec/dnssec-signzone.c:3471:13: warning: Either the condition 'ndskeys==8' is redundant or the array 'dskeyfile[8]' is accessed at index 8, which is out of bounds. [arrayIndexOutOfBoundsCond]
       dskeyfile[ndskeys++] = isc_commandline_argument;
                ^
    bin/dnssec/dnssec-signzone.c:3468:16: note: Assuming that condition 'ndskeys==8' is not redundant
       if (ndskeys == MAXDSKEYS) {
                   ^
    bin/dnssec/dnssec-signzone.c:3471:13: note: Array index out of bounds
       dskeyfile[ndskeys++] = isc_commandline_argument;
                ^

    bin/dnssec/dnssec-signzone.c:772:20: warning: Either the condition 'l->hashbuf==NULL' is redundant or there is pointer arithmetic with NULL pointer. [nullPointerArithmeticRedundantCheck]
     memset(l->hashbuf + l->entries * l->length, 0, l->length);
                       ^
    bin/dnssec/dnssec-signzone.c:768:18: note: Assuming that condition 'l->hashbuf==NULL' is not redundant
      if (l->hashbuf == NULL) {
                     ^
    bin/dnssec/dnssec-signzone.c:772:20: note: Null pointer addition
     memset(l->hashbuf + l->entries * l->length, 0, l->length);
                       ^

Instead of suppressing all such warnings individually, conditionally
define a preprocessor macro which prevents them from being triggered.