Include <signal.h> header to define SIGTERM symbol

[a-roy]

Tested using gcc version 7.2.0, Galendae does not compile because 'SIGTERM' is undeclared. The header file <signal.h> needs to be included in src/instance.c.

[chris-marsh]

Thanks a-roy. For some reason I had included SIGTERM from <sys/types.h> but this seems to have stopped working. I've changed the code as per your more sensible suggestion.