fixdep: do not ignore kconfig.h

fixdep: do not ignore kconfig.h

kconfig.h was excluded from consideration by fixdep by
6a5be57f0f00 (fixdep: fix extraneous dependencies) to avoid some false
positive hits

(1) include/config/.h
(2) include/config/h.h
(3) include/config/foo.h

(1) occurred because kconfig.h contains the string CONFIG_ in a
comment. However, since dee81e988674 (fixdep: faster CONFIG_ search), we
have a check that the part after CONFIG_ is non-empty, so this does not
happen anymore (and CONFIG_ appears by itself elsewhere, so that check
is worthwhile).

(2) comes from the include guard, __LINUX_KCONFIG_H. But with the
previous patch, we no longer match that either.

That leaves (3), which amounts to one [1] false dependency (aka stat() call
done by make), which I think we can live with:

We've already had one case [2] where the lack of include/linux/kconfig.h in
the .o.cmd file caused a missing rebuild, and while I originally thought
we should just put kconfig.h in the dependency list without parsing it
for the CONFIG_ pattern, we actually do have some real CONFIG_ symbols
mentioned in it, and one can imagine some translation unit that just
does '#ifdef __BIG_ENDIAN' but doesn't through some other header
actually depend on CONFIG_CPU_BIG_ENDIAN - so changing the target
endianness could end up rebuilding the world, minus that small
TU. Quoting Linus,

  ... when missing dependencies cause a missed re-compile, the resulting
  bugs can be _really_ subtle.

[1] well, two, we now also have CONFIG_BOOGER/booger.h - we could change
that to FOO if we care

[2] https://lkml.org/lkml/2018/2/22/838

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>