Fix gdb/data-directory Makefile-regeneration rule

My recent changes to the gdb build removed a special case for
data-directory/Makefile, but neglected to update the rule in that
Makefile.  This patch fixes the error by rewriting this rule.

gdb/ChangeLog
2021-06-05  Tom Tromey  <tom@tromey.com>

	* data-directory/Makefile.in (Makefile): Rewrite.