To prevent "wrong" musl packages which have a new version number
but the package still contains an old version, because
PKG_SOURCE_VERSION was unchanged.
Ref: musl ml https://www.openwall.com/lists/musl/2020/05/22/4
Reviewed-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Alexander Couzens <lynxis@fe80.eu>