configure: fix the check for MFX_CODEC_VP9

The data structures for VP9 in mfxvp9.h is wrapped by
MFX_VERSION_NEXT, which means those data structures have never been used
in a public release. Actually MFX_CODEC_VP9 and other VP9 stuffs are
added in mfxstructures.h. In addition, mfxdefs.h is included in
mfxvp9.h, so we may use the check in this patch for MFX_CODEC_VP9

This is in preparation for oneVPL support because mfxvp9.h is removed
from oneVPL [1]

[1]: https://github.com/oneapi-src/oneVPL

configure

@@ -6591,7 +6591,7 @@ enabled libmfx            && { { check_pkg_config libmfx "libmfx >= 1.28 libmfx
                                    "multi-frame encode, user plugins and LA_EXT rate control mode are enabled"; }
 
 if enabled libmfx; then
-   check_cc MFX_CODEC_VP9 "mfx/mfxvp9.h mfx/mfxstructures.h" "MFX_CODEC_VP9"
+   check_cc MFX_CODEC_VP9 "mfx/mfxdefs.h mfx/mfxstructures.h" "MFX_CODEC_VP9"
 fi
 
 enabled libmodplug        && require_pkg_config libmodplug libmodplug libmodplug/modplug.h ModPlug_Load