[JITTER] pvm-vm.h contains a common variable

[Jose E. Marchesi]

Hi Luca.

Jitter generates a common variable in pvm-vm.h:

struct jitter_list_header * const pvm_states;

You can see a Rawhide build log in [1].  The linker fails complaining
because of multiple definitions of `pvm_states'.

It shall be possible to include pvm-vm.h several times, so can you
please make that declaration an extern?

This is something that will surely impact every jitter user with GCC 10.

Thanks! :)

[1] 
https://build.opensuse.org/package/live_build_log/home:dancermak:poke/poke/Fedora_Rawhide/x86_64