Fix signed/unsigned comparison

It breaks a build if warnings are treated as errors.

Task-number: QTBUG-65357
Change-Id: I42d5cbdbd90f831662a6decaebecebef5005e735
Reviewed-by: Sami Nurmenniemi <sami.nurmenniemi@qt.io>
Reviewed-by: Liang Qi <liang.qi@qt.io>

src/network/kernel/qnetworkinterface_linux.cpp

@@ -158,7 +158,7 @@ template <typename Lambda> struct ProcessNetlinkRequest
     void operator()(int sock, nlmsghdr *hdr, char *buf, size_t bufsize, Lambda &&func)
     {
         // send the request
-        if (send(sock, hdr, hdr->nlmsg_len, 0) != hdr->nlmsg_len)
+        if (send(sock, hdr, hdr->nlmsg_len, 0) != ssize_t(hdr->nlmsg_len))
             return;
 
         // receive and parse the request