qsvenc: restrict MFX_RATECONTROL_LA_EXT to MFX_VERSION < 2.0

MFX_RATECONTROL_LA_EXT isn't supported for MFX_VERSION >= 2.0[1][2].
This is in preparation for oneVPL support

[1] https://spec.oneapi.io/versions/latest/elements/oneVPL/source/VPL_intel_media_sdk.html#msdk-full-name-feature-removals
[2] https://github.com/oneapi-src/oneVPL

libavcodec/qsvenc.c

@@ -141,7 +141,9 @@ static const struct {
 #if QSV_HAVE_VCM
     { MFX_RATECONTROL_VCM,     "VCM" },
 #endif
+#if !QSV_ONEVPL
     { MFX_RATECONTROL_LA_EXT,  "LA_EXT" },
+#endif
     { MFX_RATECONTROL_LA_HRD,  "LA_HRD" },
     { MFX_RATECONTROL_QVBR,    "QVBR" },
 };