fix whitespace bogon in some versions of make

With three different make binaries I have available, configuring a
pristine QEMU tree and attempting to make gives the cryptic:

Makefile:27: *** missing separator.  Stop.

This patch fixes it (presumably because it makes the output of
`set-vpath' be an empty string, rather than a bit of whitespace), but I
don't understand why this hasn't been a problem for other folks before.

Signed-off-by: Nathan Froyd <froydnj@codesourcery.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>