vendor.sh: add missing error handle on sha512sum

it's still outputting to a file, with an error handle
there, but use of x_ on the sha512sum command itself
adds further assurance of reliability.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/vendor.sh

@@ -316,7 +316,7 @@ prep()
 mksha512()
 {
 	[ "${1%/*}" != "$1" ] && x_ cd "${1%/*}"
-	sha512sum ./"${1##*/}" >> "$2" || \
+	x_ sha512sum ./"${1##*/}" >> "$2" || \
 	    err "!sha512sum \"$1\" > \"$2\"" "mksha512" "$@"
 }