kbuild: save $(strip ...) for calling any-prepreq

The string returned by $(filter-out ...) does not contain any leading
or trailing spaces.

So, only the space that matters is the one between

  $(filter-out $(PHONY),$?)

and

  $(filter-out $(PHONY) $(wildcard $^),$^)

By removing it from the code, we can save $(strip ...) evaluation.
This refactoring is possible because $(any-prereq) is only passed to
the first argument of $(if ...), so we are only interested in whether
or not it is empty.

This is also the prerequisite for the next commit.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>