ssh: fix narrowing conversion warning

'left' is used as time_t but declared as long.
MinGW complains:
error: conversion to 'long int' from 'time_t {aka long long int}' may alter
its value [-Werror=conversion]
Changed the declaration to time_t.

lib/ssh.c

@@ -2822,7 +2822,7 @@ static CURLcode ssh_block_statemach(struct connectdata *conn,
 
   while((sshc->state != SSH_STOP) && !result) {
     bool block;
-    long left;
+    time_t left;
     struct timeval now = Curl_tvnow();
 
     result = ssh_statemach_act(conn, &block);