sstate.bbclass: Enclose sstate search parameter with quotes

In case filenames have spaces, execution of the function
sstater_install will hang, so the print parameter %s must be
enclosed with quotes.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@freescale.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>