ngt_RandomStr(): Add implicit cast to "unsigned".

This fixes the following warning of Xcode 4.5:

 src/tool/tool.c:150:19:
  Implicit conversion loses integer precision: 'long' to 'unsigned int'

src/tool/tool.c

@@ -147,7 +147,7 @@ ngt_RandomStr( char *String, const size_t len)
 
 	struct timeval t;
 	gettimeofday(&t, NULL);
-	srand(t.tv_usec * t.tv_sec);
+	srand((unsigned)(t.tv_usec * t.tv_sec));
 
 	for (size_t i = 0; i < len; ++i) {
 		String[i] = chars[rand() % (sizeof(chars) - 1)];