USE __COUNTER__ instead of __LINE__ when available

USE __COUNTER__ instead of __LINE__ when available

I've seen builds fail with

nm-connection.c:119:691: error: declaration of '_GStaticAssertCompileTimeAssertion_119' shadows a previous local [-Werror=shadow]

because several compile-time assertions ended up on the same
line. __COUNTER__ is meant specifically for the purpose of
constructing identifiers, so use it when available.