set the seed for the RNG just once at the beginning of the program

this improves randomness for tests where createNumbers() is called in rapid succession

main.c

@@ -1,5 +1,6 @@
 #include <stdlib.h>
 #include <stdio.h>
+#include <time.h>
 #include "numbers.h"
 #include "timer.h"
 #include "highscore.h"
@@ -39,6 +40,9 @@ int main(int argc, char *argv[])
 {
     int exitCode = EXIT_FAILURE;
 
+    // set seed
+    srand(time(NULL));
+
     if(argc != 2)
     {
         fprintf(stderr, "Usage: %s <player name>\n", argv[0]);