Makefile: add missing dependencies of 'config-list.h'

Makefile: add missing dependencies of 'config-list.h'

We auto-generate the list of supported configuration variables from
'Documentation/config/*.txt', and that list used to be created by the
'generate-cmdlist.sh' helper script and stored in the 'command-list.h'
header.  Commit 709df95b78 (help: move list_config_help to
builtin/help, 2020-04-16) extracted this into a dedicated
'generate-configlist.sh' script and 'config-list.h' header, and added
a new target in the 'Makefile' as well, but while doing so it forgot
to extract the dependencies of the latter.  Consequently, since then
'config-list.h' is not re-generated when 'Documentation/config/*.txt'
is updated, while 'command-list.h' is re-generated unnecessarily:

  $ touch Documentation/config/log.txt
  $ make -j4
      GEN command-list.h
      CC help.o
      AR libgit.a

Fix this and list all config-related documentation files as
dependencies of 'config-list.h' and remove them from the dependencies
of 'command-list.h'.

  $ touch Documentation/config/log.txt
  $ make
      GEN config-list.h
      CC builtin/help.o
      LINK git

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>