Fix FreeBSD build

Fixed swim headers in addition to commit:
88892f13 ('swim: fix build on FreeBSD')

Included unistd.h header to fix the following errors:

In file included from src/lib/swim/swim_transport_udp.c:31:
src/lib/swim/swim_transport.h:61:1:
    error: unknown type name 'ssize_t'; did you mean 'size_t'?
ssize_t
^~~~~~~
size_t
/usr/include/netinet6/in6.h:698:18: note: 'size_t' declared here
typedef __size_t        size_t;
                        ^
In file included from src/lib/swim/swim_transport_udp.c:31:
src/lib/swim/swim_transport.h:66:1:
    error: unknown type name 'ssize_t'; did you mean 'size_t'?
ssize_t
^~~~~~~
size_t
/usr/include/netinet6/in6.h:698:18: note: 'size_t' declared here
typedef __size_t        size_t;

Closes #4050

src/lib/swim/swim_transport.h

@@ -32,6 +32,7 @@
  */
 #include <netinet/in.h>
 #include <arpa/inet.h>
+#include <unistd.h>
 
 struct ifaddrs;