BUILD: peers: fix build warning with gcc 4.2.1

BUILD: peers: fix build warning with gcc 4.2.1

Building on OpenBSD 6.7 with gcc-4.2.1 yields the following warnings
which suggest that the initialization is not taken as expected but
that the container member is reset with each initialization:

  src/peers.c: In function 'peer_send_updatemsg':
  src/peers.c:1000: warning: initialized field overwritten
  src/peers.c:1000: warning: (near initialization for 'p.updt')
  src/peers.c:1001: warning: initialized field overwritten
  src/peers.c:1001: warning: (near initialization for 'p.updt')
  src/peers.c:1002: warning: initialized field overwritten
  src/peers.c:1002: warning: (near initialization for 'p.updt')
  src/peers.c:1003: warning: initialized field overwritten
  src/peers.c:1003: warning: (near initialization for 'p.updt')
  src/peers.c:1004: warning: initialized field overwritten
  src/peers.c:1004: warning: (near initialization for 'p.updt')

Fixing this is trivial, we just have to initialize one level at
a time.