[fuzzing] Enable FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION

The fuzzing build macro was elided at some point in the development of
the fuzz target GN tempate.  This CL adds it back.

SEC-44

Tests: Confirmed flag is set on ninja invocation for fuzz targets
Change-Id: I22f141f5c7a606fae0f3f11b5b0acc5e95445fc8
2 files changed