kbuild: do not pass $(KBUILD_CFLAGS) to scripts/mkcompile_h

scripts/mkcompile_h uses $(CC) only for getting the version string.

I suspected there was a specific reason why the additional flags were
needed, and dug the commit history. This code dates back to at least
2002 [1], but I could not get any more clue.

Just get rid of it.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=29f3df7eba8ddf91a55183f9967f76fbcc3ab742

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>