third_party/heimdal_build: Don't generate .x source files

third_party/heimdal_build: Don't generate .x source files

This is an adaptation to Heimdal:

commit 9427796f1a65906f12768b28abdb5a928222f3c6
Author: Jeffrey Altman <jaltman@secure-endpoints.com>
Date:   Wed Jan 5 15:45:23 2022 -0500

    Generate .x source files as .c source files

    The generated .x source and .hx header files are plain C source files.
    Generate them as .c source files and avoid unnecessary file copying
    and special makefile rules.

    Change-Id: Ifc4bbe3c46dd357fdd642040ad964c7cfe1d395c

NOTE: THIS COMMIT WON'T COMPILE/WORK ON ITS OWN!

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14995

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>