makedepends_x86_64: switch back vmaf-git to vmaf

At the time of ffmpeg 4.0 release, the latest version of vmaf was
1.3.1. At that time, ffmpeg 4.0 needed vmaf git master to compile.

Now vmaf 1.3.4 was released and ffmpeg builds fine with it.

PKGBUILD

@@ -4,7 +4,7 @@
 pkgname=ffmpeg-full
 _srcname=ffmpeg
 pkgver=4.0
-pkgrel=3
+pkgrel=4
 pkgdesc='Complete solution to record, convert and stream audio and video (all possible features including nvenc, qsv and libfdk-aac)'
 arch=('i686' 'x86_64')
 url='http://www.ffmpeg.org/'
@@ -41,7 +41,7 @@ makedepends=(
 )
 makedepends_x86_64=(
     # AUR:
-        'vmaf-git'
+        'vmaf'
 )
 provides=(
     'ffmpeg' 'ffmpeg-full-nvenc' 'ffmpeg-nvenc' 'ffmpeg-libfdk_aac' 'ffmpeg-decklink'