selftests/vDSO: remove partially duplicated "all:" target in Makefile

There were a couple of errors here:

1. TEST_GEN_PROGS was incorrectly prepending $(OUTPUT) to each program
to be built. However, lib.mk already does that because it assumes "bare"
program names are passed in, so this ended up creating
$(OUTPUT)/$(OUTPUT)/file.c, which of course won't work as intended.

2. lib.mk was included before TEST_GEN_PROGS was set, which led to
lib.mk's "all:" target not seeing anything to rebuild.

So nothing worked, which caused the author to force things by creating
an "all:" target locally--while still including ../lib.mk.

Fix all of this by including ../lib.mk at the right place, and removing
the $(OUTPUT) prefix to the programs to be built, and removing the
duplicate "all:" target.

Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>