parisc: Add missing FORCE prerequisites in Makefile

Fix those make warnings:
    arch/parisc/kernel/vdso32/Makefile:30: FORCE prerequisite is missing
    arch/parisc/kernel/vdso64/Makefile:30: FORCE prerequisite is missing

Add the missing FORCE prerequisites for all build targets identified by
"make help".

Fixes: e1f86d7b

 ("kbuild: warn if FORCE is missing for if_changed(_dep,_rule) and filechk")
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: <stable@vger.kernel.org> # 5.18+