src: only block SIGUSR1 when HAVE_INSPECTOR

I'm currently seeing a timeout error for test-signal-handler.js on
macosx when using the following configuration:

./configure --debug --without-ssl && make -j8 test

--without-ssl implies that there will be no inspector but the signal
SIGUSR1 is blocked in PlatformInit just the same. But in this case
never unblocked which is causing the signal to never be delivered to
the handlers in test-signal-handler.js and it loops until it times out.

Not sure if this is the best way of fixing this but hopefully more eyes
on this will help.

PR-URL: https://github.com/nodejs/node/pull/12266


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>