Add extern C declaration to schedule.h
Add extern C declaration to schedule.h

All headers except the recently added schedule.h header include an extern C
declaration. Add the same declaration to schedule.h to allow flawless inclusion
in C++ programs.

Signed-off-by: Tobias Grosser <059b8b880f8441509ec8a65b50b4c6ae74ebea76@grosser.es>
Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

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