selftest/prservice.py: Sanitize package version when looking for stamp

selftest/prservice.py: Sanitize package version when looking for stamp

Currently when using a git version the check for the stamp, using regex,
will fail because of plus sign in the version.

With this change the version is escaped before adding it to the regex.

(From OE-Core rev: 1aefa6a4dec84a5581aab70451bb84801b3b3615)

Signed-off-by: Mariano Lopez <mariano.lopez@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>