composite: log when disabled via OpenGLIsUnsafe

After updating my Debian buster to bullseye, I noticed missing
shadows / composite for KDE, which made overlapping windows hard
to recognize.

.xsession-errors just had "kwin_core: Compositing is not possible".
Using the suggested QT_LOGGING_RULES="kwin_*.debug=true" didn't
provide any more clues; nothing I tried did.
Eventually I ended reading the kwin source and found, that
disabled composite via kwinrc :: [Compositing] :: OpenGLIsUnsafe
was not reported.

IMHO all these disabling cases should use qCCritical(KWIN_CORE),
like the "Compositing is not possible" message, but for the time
being, this simply logs the OpenGLIsUnsafe reason, just like all
the others.