Fix Tru64 socklen_t compile issue with example/direct_tcpip.c

Building libssh2-1.2.3 on Tru64 fails at line 48 and 166 because socklen_t
isn't defined on Tru64 unless _POSIX_PII_SOCKET is defined.

This patch updates configure.ac to add -D_POSIX_PII_SOCKET when building
on Tru64 platform(s).

configure.ac

@@ -41,6 +41,9 @@ case "$host" in
 	;;
     *hpux*)
     ;;
+    *osf*)
+    CFLAGS="$CFLAGS -D_POSIX_PII_SOCKET"
+    ;;
     *)
     ;;
 esac