kbuild: fix Kconfig prompt of CONFIG_HEADERS_CHECK

Prior to commit 257edce6 ("kbuild: exploit parallel building for
CONFIG_HEADERS_CHECK"), the sanity check of exported headers was done
as a side-effect of build rule of vmlinux.

That commit is good, but I missed to update the prompt of the Kconfig
entry. For the sake of preciseness, lets' say "when building 'all'".

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