Disable liblzma/xz symbol versioning

Does not make a whole lot of sense for static builds to begin with,
and since 913ddc5572
it's outright broken for unknown reasons:

ld: libavcodec/libavcodec.so.59: version node not found for symbol lzma_get_progress@XZ_5.2.2
ld: failed to set dynamic section sizes: bad value

scripts.d/25-xz.sh

@@ -15,6 +15,7 @@ ffbuild_dockerbuild() {
 
     local myconf=(
         --prefix="$FFBUILD_PREFIX"
+        --disable-symbol-versions
         --disable-shared
         --enable-static
         --with-pic