heimdal: Fix build with system provided heimdal library

heimdal: Fix build with system provided heimdal library

Trying to compile with a system provided heimdal library
results in this compile error:

[ 876/3043] Compiling source4/auth/kerberos/srv_keytab.c
In file included from /usr/include/heimdal/krb5.h:949:0,
                 from ../lib/replace/system/kerberos.h:33,
                 from ../source4/auth/kerberos/srv_keytab.c:31:
/usr/include/heimdal/krb5-protos.h:3894:1: error: unknown type name ‘HEIMDAL_WARN_UNUSED_RESULT_ATTRIBUTE’; did you mean ‘_WARN_UNUSED_RESULT_’?
 HEIMDAL_WARN_UNUSED_RESULT_ATTRIBUTE KRB5_LIB_FUNCTION krb5_error_code KRB5_LIB_CALL
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 _WARN_UNUSED_RESULT_
/usr/include/heimdal/krb5-protos.h:3895:1: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘krb5_generate_random’
 krb5_generate_random (

The problem is that Samba provides a minimal krb5-types.h file
for the internal build that gets used during the build with
the system provided heimdal library. As the minimal file
does not provide all definitions, the build fails.

Fix this by having the krb-types.h file simply include the
include file from the system library, if the build is done
using the system provided heimdal library.

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Tue Jul 10 07:30:26 CEST 2018 on sn-devel-144