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>

configure

@@ -4347,7 +4347,7 @@ enabled libwebp           && require_pkg_config libwebp webp/encode.h WebPGetEnc
 enabled libx264           && require_pkg_config x264 "stdint.h x264.h" x264_encoder_encode &&
                              { check_cpp_condition x264.h "X264_BUILD >= 118" ||
                                die "ERROR: libx264 version must be >= 0.118."; }
-enabled libx265           && require_pkg_config x265 x265.h x265_encoder_encode &&
+enabled libx265           && require_pkg_config x265 x265.h x265_api_get &&
                              { check_cpp_condition x265.h "X265_BUILD >= 57" ||
                                die "ERROR: libx265 version must be >= 57."; }
 enabled libxavs           && require libxavs xavs.h xavs_encoder_encode -lxavs