wscript: Fix build with system heimdal

wscript: Fix build with system heimdal

Ubuntu has heimdal include files in /usr/include/heimdal. As the
kerberos include files are pulled into many files through indirect
includes, add the discovered include paths to EXTRA_INCLUDES to
always have them available.

Also set USING_SYSTEM_KRB5 when enabling the system heimdal build,
to correctly handle the inclusion of the krb5-types.h file.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14179

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>