kcsan: Silence -Wmissing-prototypes warning with W=1

The functions here should not be forward declared for explicit use
elsewhere in the kernel, as they should only be emitted by the compiler
due to sanitizer instrumentation.  Add forward declarations a line above
their definition to shut up warnings in W=1 builds.

Link: https://lkml.kernel.org/r/202006060103.jSCpnV1g%lkp@intel.com


Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>