Fix RBC incremental builds

Kati regenerates its ninja file if an environment variable
that was referenced by the makefiles was changed since the
last run. TRACE_BEGIN_SOONG is a constantly changing variable
that was referenced by dump-variables-rbc, which references
all all-caps variables, with a few exceptions.

Add TRACE_BEGIN_SOONG to that exception list so kati doesn't
rerun every time.

Fixes: 216531048
Test: Manually
Change-Id: I2df65b6f6aa968f132380e3410763d907d9e3e0f