kernel: Add missing marshalling header for k_reschedule

[thughes]

Without this header, compiling the kernel.poll test with -Werror=unused-function fails.

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[andyross]

Hm... how is it not using the function? This should be followed by an include of the generated "...mrsh.c" marshalling code which references it (the marshalling function then gets stuffed into the global table of syscalls handlers).

I suspect the real problem is elsewhere. Did the include get forgotten?

[andyross]

Yeah, that seems like it. Just copy over the marshalling boilerplate from another function. This is a common goof; we'll add a syscalls and test coverage for it, but then forget to have coverage calling it from a userspace process.