Remove indirection in stdio-common tests dependencies.

stdio-common/Makefile has the tests target depend on two test output
files indirectly through rules do-tst-unbputc and do-tst-printf.  I
see no reason for such an indirection, and everywhere else the tests
just depend on the output files directly, so this patch removes the
indirection.

Tested x86_64.

	* stdio-common/Makefile (do-tst-unbputc): Remove target.
	(do-tst-printf): Likewise.
	(tests): Depend directly on $(objpfx)tst-unbputc.out and
	$(objpfx)tst-printf.out.