sparc: vdso: add FORCE to the build rule of %.so

$(call if_changed,...) must have FORCE as a prerequisite.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>