Introduce nob_touch_file()

[mikmart]

I found this useful while trying to debug #111 with a program like:

#define NOB_IMPLEMENTATION
#define NOB_EXPERIMENTAL_DELETE_OLD
#include "../nob.h"

int main(int arg, char **argv) {
    NOB_GO_REBUILD_URSELF(arg, argv);
    nob_log(NOB_INFO, "Hello, World!");
    nob_touch_file(__FILE__);
    return 0;
}

Maybe it's useful in general?