Makefile: fix race if we run `make update-mocks` in a clean tree.

/bin/sh: 1: ccan/ccan/cdump/tools/cdump-enumstr: Text file busy
    make[1]: *** [common/Makefile:81: common/gen_htlc_state_names.h] Error 2
    make[1]: *** Waiting for unfinished jobs....

The fix is to make sure all generated headers are made first, and
thus cdump-enumstr.

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

Makefile

@@ -430,6 +430,7 @@ clean:
 	find . -name '*gcno' -delete
 	find . -name '*.nccout' -delete
 
+update-mocks: $(ALL_GEN_HEADERS)
 update-mocks/%: %
 	@MAKE=$(MAKE) tools/update-mocks.sh "$*"