compiler.h, seqlock.h: Remove unnecessary kcsan.h includes

No we longer have to include kcsan.h, since the required KCSAN interface
for both compiler.h and seqlock.h are now provided by kcsan-checks.h.

Acked-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>