src: fix building --without-v8-platform (backport v7.x)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

src

Backport of the relevant commit from #11088 to v7.x-staging.

The call signature of v8_platform.StartInspector needs to be the same whether or not NODE_USE_V8_PLATFORM, otherwise Node will fail to compile if HAVE_INSPECTOR and !NODE_USE_V8_PLATFORM.

(cherry picked from commit e619725f)