x264enc: Let libx264 choose the keyframe type on force-key-unit

Open Arun Raghavan requested to merge arun/gstreamer:keyframe-type into main

Without this, when forcing a keyframe with closed GOPs, we see warnings like:

  WARN                 x264enc gstx264enc.c:1274:gst_x264_enc_log_callback:<x264enc0> forced frame type (1) at 60 was changed to frame type (2)

This may or may not be a problem in actual, depending on the configured key frame interval, but it seems more reasonable to let the library make the right choice of keyframe type than to force it.

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