When CONFIG_DEBUG_INFO_BTF_MODULES=y, running 'make modules'
in the clean kernel tree will get the following error.

  $ grep CONFIG_DEBUG_INFO_BTF_MODULES .config
  CONFIG_DEBUG_INFO_BTF_MODULES=y
  $ make -s clean
  $ make modules
    [snip]
    AR      vmlinux.a
  ar: ./built-in.a: No such file or directory
  make: *** [Makefile:1241: vmlinux.a] Error 1

'modules' depends on 'vmlinux', but builtin objects are not built.

Define KBUILD_BUILTIN.

Fixes: f73edc89 ("kbuild: unify two modpost invocations")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>