BUILD: ssl: include buffer common headers for ssl_sock_ctx

BUILD: ssl: include buffer common headers for ssl_sock_ctx

since commit c0cdaffaa338 ("REORG: ssl: move ssl_sock_ctx and fix
cross-dependencies issues"), `struct ssl_sock_ctx` was moved in
ssl_sock.h. As it contains a `struct buffer`, including
`common/buffer.h` is now mandatory. I encountered an issue while
including ssl_sock.h on another patch:

include/types/ssl_sock.h:240:16: error: field ‘early_buf’ has incomplete type
  240 |  struct buffer early_buf;      /* buffer to store the early data received */

no backport needed.

Fixes: c0cdaffaa338 ("REORG: ssl: move ssl_sock_ctx and fix
cross-dependencies issues")
Signed-off-by: William Dauchy <w.dauchy@criteo.com>