tcpclientsrc, tcpserversrc: fix tcp stats gathering

Fix-up after commit e56b78417. Unclear how that could
ever have worked. Code only built because basically
everything was disabled due to the missing config.h
include.

- Include config.h so that HAVE_* defines are available.
- Fix missing variable declarations that somehow disappeared
  when moving the stats gathering code block into a separate
  function in a new file.
- Fix structure variable modified to match name in new function.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8211>

subprojects/gst-plugins-base/gst/tcp/gsttcpsrcstats.c

@@ -1,3 +1,7 @@
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
 /* macOS and iOS have the .h files but the tcp_info struct is private API */
 #if defined(HAVE_NETINET_TCP_H) && defined(HAVE_NETINET_IN_H) && defined(HAVE_SYS_SOCKET_H)
 #include <netinet/tcp.h>
@@ -14,10 +18,14 @@ void
 gst_tcp_stats_from_socket (GstStructure * structure, GSocket * socket)
 {
 #ifdef HAVE_SOCKET_METRIC_HEADERS
+  struct tcp_info info = { 0, };
+  socklen_t info_len = sizeof (info);
+  int fd = g_socket_get_fd (socket);
+
   if (getsockopt (fd, IPPROTO_TCP, TCP_INFO, &info, &info_len) == 0) {
     /* this is system-specific */
 #ifdef HAVE_BSD_TCP_INFO
-    gst_structure_set (s,
+    gst_structure_set (structure,
         "reordering", G_TYPE_UINT, info.__tcpi_reordering,
         "unacked", G_TYPE_UINT, info.__tcpi_unacked,
         "sacked", G_TYPE_UINT, info.__tcpi_sacked,
@@ -25,7 +33,7 @@ gst_tcp_stats_from_socket (GstStructure * structure, GSocket * socket)
         "retrans", G_TYPE_UINT, info.__tcpi_retrans,
         "fackets", G_TYPE_UINT, info.__tcpi_fackets, NULL);
 #elif defined(HAVE_LINUX_TCP_INFO)
-    gst_structure_set (s,
+    gst_structure_set (structure,
         "reordering", G_TYPE_UINT, info.tcpi_reordering,
         "unacked", G_TYPE_UINT, info.tcpi_unacked,
         "sacked", G_TYPE_UINT, info.tcpi_sacked,