Fixed build with signed socklen_t and unix sockets.

This seems to be the case at least under Cygwin, where build was broken
by 05ba5bce31e0 (1.5.3).

Reported by Kevin Worthington,
http://mailman.nginx.org/pipermail/nginx/2013-August/040028.html.

src/core/ngx_inet.c

diff --git a/src/core/ngx_inet.c b/src/core/ngx_inet.c
index 2b5c94c..bb03720 100644
--- a/src/core/ngx_inet.c
+++ b/src/core/ngx_inet.c
@@ -233,7 +233,7 @@
 
         /* on Linux sockaddr might not include sun_path at all */
 
-        if (socklen <= offsetof(struct sockaddr_un, sun_path)) {
+        if (socklen <= (socklen_t) offsetof(struct sockaddr_un, sun_path)) {
             p = ngx_snprintf(text, len, "unix:%Z");
 
         } else {