Win32: fixed build with MinGW and MinGW-w64 gcc.

This change fixes the "comparison between signed and unsigned integer
expressions" warning, introduced in 5e6142609e48 (1.9.4).

src/core/ngx_connection.c

diff --git a/src/core/ngx_connection.c b/src/core/ngx_connection.c
index 9f2675b..0c19d5d 100644
--- a/src/core/ngx_connection.c
+++ b/src/core/ngx_connection.c
@@ -1175,7 +1175,7 @@
 
         /* THREAD: lock */
 
-        if (c[i].fd != -1 && c[i].idle) {
+        if (c[i].fd != (ngx_socket_t) -1 && c[i].idle) {
             c[i].close = 1;
             c[i].read->handler(c[i].read);
         }