configure: Check for x265_api_get

Any other x265 symbol may not exported, e.g. if the build is a
multilib (10-bit and 8-bit in one) build.

This is the only symbol we directly call, and is available in the
build number we check for.

Fixes the configure check on multilib x265 builds.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>