Fix building with ISO C90

This fixes the changes of commit 574bd6e5aa

src/core/tcp_in.c

@@ -1687,6 +1687,7 @@ tcp_receive(struct tcp_pcb *pcb)
                  ->ooseq. We check the lengths to see which one to
                  discard. */
               if (inseg.len > next->len) {
+                struct tcp_seg* cseg;
 
                 /* If next segment is the last segment in ooseq
                    and smaller than inseg, that means it has been
@@ -1699,7 +1700,7 @@ tcp_receive(struct tcp_pcb *pcb)
                 /* The incoming segment is larger than the old
                    segment. We replace some segments with the new
                    one. */
-                struct tcp_seg *cseg = tcp_seg_copy(&inseg);
+                cseg = tcp_seg_copy(&inseg);
                 if (cseg != NULL) {
                   if (prev != NULL) {
                     prev->next = cseg;