x264enc: support X264 build <= 155

Bug added in PR [1].
The X264_CSP_I400 def was included in the build 156 [2].
Ubuntu 20.04 LTS contains the build 155 [3].

[1] !4812 (merged)
[2] https://github.com/mirror/x264/commit/698c5a32e63
[3] https://packages.ubuntu.com/source/focal/x264