lib/string_map: fix return type of string_cmp

I can't figure out how checking the sign of a bool ever worked. The
following program demonstrates the problem (i.e. for me it prints 1).

 #include <stdio.h>
 #include <stdbool.h>
 int main(int argc, char **argv) {
    bool x;
    x = -1;
    printf("x = %d\n", x);
 }

This seems to be mandated by the C99 standard 6.3.1.2.