user/fftw: undo test version

user/fftw/APKBUILD

 # Contributor: William Pitcock <nenolod@dereferenced.org>
 # Maintainer: 
 pkgname=fftw
-pkgver=3.3.6p2
+pkgver=3.3.8
 _pkgver=$(printf '%s' "$pkgver" | sed 's/p/-pl/')
 pkgrel=0
 pkgdesc="Fastest fourier transform in the west"