jool-kmod: Fix S trailing slash build warning

This fixes the following warning:

WARNING:
[...]jool-kmod_git.bb: Recipe jool-kmod sets S variable with trailing
slash '[...]/jool-kmod/git-r0/git/src/mod/', remove it

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>