xkbcomp: downgrade "Symbol added to modifier map for multiple modifiers" log to a...

xkbcomp: downgrade "Symbol added to modifier map for multiple modifiers" log to a warning

This condition happens in xkeyboard-config keymaps and seems hard to
fix. Currently it incessantly spams people's logs who have no idea what
to do about it. So downgrade to "warning" level, so it doesn't show up
by default.

When working on keymaps, set `XKB_LOG_LEVEL=debug XKB_LOG_VERBOSITY=10`
to see all possible messages.

Refs https://github.com/xkbcommon/libxkbcommon/issues/111
Fixes https://github.com/xkbcommon/libxkbcommon/issues/128
Signed-off-by: Ran Benita <ran@unusedvar.com>