Add some missing includes.

Add some missing includes.

Letting .c files include their "own" header file guarantees that the
declaration in the header file stays in sync with the definition.

Eventually we may even enable gcc's -Wmissing-declarations, but
we're not there yet, mainly due to command handlers, init functions
and completers.

Note that including signal.h in signal.c required two other includes
since the header file defines a task structure.