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.

Co-developed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Jann Horn <jannh@google.com>
[kees: merged similar static-ness fixes into a single patch]
Link: https://lkml.kernel.org/r/20190326230841.87834-1-jannh@google.com
Link: https://lkml.kernel.org/r/1553673018-19234-1-git-send-email-mojha@codeaurora.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.morris@microsoft.com>