Fix the restart button

Hello,

I have stolen this patch from OpenBSD, which was done to please pledge, but in
the end fixes the restart button.

https://bapt.nours.eu/patch-xpcom_build_BinaryPath.h

If my investigation is right, firefox when one click on the restart button (in
case of upgrade) will call for the sysctl KERN_PROC_PATHNAME, which will return
en empty value, because the binary has been unlinked and replace by the new one.
(the behaviour is different on linux here) but forcing the path at buildtime
make it work reliably and at least here, it makes the restart button working
reliably

note that it needs a post-patch target to replace %%PREFIX%% in
xpcom/build/BinaryPath.h file.

Best regards,
Bapt