kbuild: replace $(if A,A,B) with $(or A,B) in scripts/Makefile.modpost

Similar cleanup to commit 5c816641

 ("kbuild: replace $(if A,A,B)
with $(or A,B)").

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>