configure: add dxva2 extralibs to avcodec

DXVA2 may be enabled even when every relevant module is disabled,
which would result in the dependency generator not including its
extralibs to avcodec.

Fixes ticket #7642.
Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -3545,7 +3545,7 @@ swresample_suggest="libm libsoxr"
 swscale_deps="avutil"
 swscale_suggest="libm"
 
-avcodec_extralibs="pthreads_extralibs iconv_extralibs"
+avcodec_extralibs="pthreads_extralibs iconv_extralibs dxva2_extralibs"
 avfilter_extralibs="pthreads_extralibs"
 avutil_extralibs="d3d11va_extralibs nanosleep_extralibs pthreads_extralibs vaapi_drm_extralibs vaapi_x11_extralibs vdpau_x11_extralibs"