build: output-example: Add avutil to ELIBS in link command

output-example links against swscale, which depends on avutil.  In standard
configurations, by pure good luck, the libs before swscale pull in all of
swscale's dependencies and linking succeeds.  However, in some configurations
like --disable-asm this is not the case and linking fails.

Hardcode the dependency to avoid a more general Makefile refactoring.