net: include timer.h where the TCP connection macros need it

tcp_conn_set_lifetime() and tcp_conn_set_msg_read_timeout() in
net/tcp_conn_defs.h call get_ticks(), but the header does not include
timer.h.  With the default Makefile.conf it arrives anyway, through
mem/shm_mem.h -> shm_mem_dbg.h -> lib/dbg/struct_hist.h, but that path
exists only with DBG_MALLOC.  test/fuzz/oss-fuzz-build.sh disables
DBG_MALLOC, so since net/proxy_protocol.c started calling
tcp_conn_reset_lifetime() (4e2758f) the fuzz build stops there with
clang:

  net/proxy_protocol.c:201:3: error: call to undeclared function
  'get_ticks'; ISO C99 and later do not support implicit function
  declarations [-Wimplicit-function-declaration]

Include timer.h in the header that uses it.

(cherry picked from commit fa98925)

Author: Lt-Flash

net/tcp_conn_defs.h

@@ -38,6 +38,7 @@
 
 #include "../locking.h"
 #include "../ip_addr.h"
+#include "../timer.h"
 #include "tcp_conn_profile.h"
 #include "proto_tcp/tcp_common_defs.h"