packaged-staging.bbclass: removed two few superfluous slashes

packaged-staging.bbclass: removed two few superfluous slashes

${SYSROOT_DESTDIR} ends on a / but there is also a / after the variable
and a slash as the first character of the next variable.
This patch removes the / that is explicitly added so in the generated
code we get // instead of the even sillier looking (and probably more confusing) ///

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>