Makefile: fix race when checking manpages.

By not depending on plugins, we can call lightningd before plugins are
built, and not get all options:

    diff of command names vs manpage names:
    --- /dev/fd/63  2019-08-22 05:06:55.265659216 +0000
    +++ /dev/fd/62  2019-08-22 05:06:55.265659216 +0000
    @@ -3,6 +3,8 @@
     allow-deprecated-apis=
     always-use-proxy=
     announce-addr=
    +autocleaninvoice-cycle=
    +autocleaninvoice-expired-by=
     autolisten=
     bind-addr=
     bitcoin-cli=

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/Makefile

@@ -78,7 +78,8 @@ maintainer-clean: doc-maintainer-clean
 clean: doc-clean
 check: check-manpages
 
-check-manpages: cli/lightning-cli lightningd/lightningd
+# This needs plugins, too.
+check-manpages: all-programs
 	@tools/check-manpage.sh cli/lightning-cli doc/lightning-cli.1.md
 	@tools/check-manpage.sh "lightningd/lightningd --lightning-dir=/tmp/" doc/lightningd-config.5.md