Yama: mark local symbols as static

sparse complains that Yama defines functions and a variable as non-static
even though they don't exist in any header. Fix it by making them static.

Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: James Morris <james.morris@microsoft.com>