fix demos/sslecho/main.c so it builds on OpenBSD too

fix demos/sslecho/main.c so it builds on OpenBSD too

trying to build `demos/sslecho/main.c` shipped by current openssl
fails with error as follows:
```
cc -I../../include -g -Wall   -c -o main.o main.c
main.c:35:24: error: variable has incomplete type 'struct sockaddr_in'
    struct sockaddr_in addr;
                       ^
main.c:35:12: note: forward declaration of 'struct sockaddr_in'
    struct sockaddr_in addr;
           ^
main.c:46:32: error: use of undeclared identifier 'INADDR_ANY'
        addr.sin_addr.s_addr = INADDR_ANY;
                               ^
main.c:152:24: error: variable has incomplete type 'struct sockaddr_in'
    struct sockaddr_in addr;
                       ^
main.c:152:12: note: forward declaration of 'struct sockaddr_in'
    struct sockaddr_in addr;
           ^
3 errors generated.
gmake: *** [<builtin>: main.o] Error 1
```

including `netinet/in.h` fixes the build

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23936)