tcp_wrappers: header file tcpd.h doesn't guard against multiple inclusion, potentially causing build errors in dependents

[mascguy (Christopher Nielsen)]

While one might argue that dependent projects should avoid including the same header multiple times, that's not always easy to avoid. Hence the reason why headers include guards, preventing errors when that occurs.

One real-world example is openldap 2.6.x, when variant +wrappers is enabled:

In file included from util-int.c:950:
/opt/local/include/tcpd.h:20:16: error: redefinition of 'sockaddr_gen'
typedef struct sockaddr_gen {
               ^
../../include/lutil.h:24:11: note: '/opt/local/include/tcpd.h' included multiple times, additional include site here
# include <tcpd.h>
          ^
util-int.c:950:11: note: '/opt/local/include/tcpd.h' included multiple times, additional include site here
# include <tcpd.h>
          ^
/opt/local/include/tcpd.h:20:16: note: unguarded header; consider using #ifdef guards or #pragma once
typedef struct sockaddr_gen {
               ^

Patch said header, to include a guard.

[Christopher Nielsen <mascguy@…>]

In 2085880447a0c5e2c3b5e0616f37a836cb03244f/macports-ports (master):

tcp_wrappers: add header guard
Fixes: #65558

[Christopher Nielsen <mascguy@…>]

In 69a08ad8128af5e055dcf04c714e4fb5d214925b/macports-ports (master):

openldap-devel: enable wrappers by default

• Now possible, with tcp_wrappers header guard added

See: #65558