Fix compiler warnings when in C99 standards mode

Output when compiling (with `clang -Wall -std=c99`):

    src/linenoise.c:201:14: warning: implicit declaration of function 'strcasecmp' is invalid
          in C99 [-Wimplicit-function-declaration]
            if (!strcasecmp(term,unsupported_term[j])) return 1;
                 ^
    src/linenoise.c:647:55: warning: implicitly declaring C library function 'strdup' with
          type 'char *(const char *)'
            history[history_len - 1 - l->history_index] = strdup(l->buf);
                                                          ^
    src/linenoise.c:647:55: note: please include the header <string.h> or explicitly provide a
          declaration for 'strdup'

`strcasecmp` just required a include of `strings.h`, but it turns out `strdup` isn't standard C - [it's a POSIX thing](http://stackoverflow.com/a/252802/716118).