configure: Fix libvmaf name

Fixes:
    fatal error: libvmaf.h: No such file or directory.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

configure

@@ -3188,7 +3188,7 @@ uspp_filter_deps="gpl avcodec"
 vaguedenoiser_filter_deps="gpl"
 vidstabdetect_filter_deps="libvidstab"
 vidstabtransform_filter_deps="libvidstab"
-vmaf_filter_deps="libvmaf"
+libvmaf_filter_deps="libvmaf"
 zmq_filter_deps="libzmq"
 zoompan_filter_deps="swscale"
 zscale_filter_deps="libzimg"