install Makefile target should depend on all, not mosquitto

This ensures that man pages are always built, even if a user runs `make
install` without building first.

Closes #1989. Thanks to woodz-.

ChangeLog.txt

@@ -13,6 +13,10 @@ Broker:
  before a role that was attached to the group or client is deleted.
  Closes #1998.
 
+Build:
+- `install` Makefile target should depend on `all`, not `mosquitto`, to ensure
+ that man pages are always built. Closes #1989.
+
 Apps:
 - Disallow control characters in mosquitto_passwd usernames.
 - Fix incorrect description in mosquitto_ctrl man page. Closes #1995.

Makefile

@@ -87,7 +87,7 @@ ptest : mosquitto
 utest : mosquitto
  $(MAKE) -C test utest
 
-install : mosquitto
+install : all
  set -e; for d in ${DIRS}; do $(MAKE) -C $${d} install; done
 ifeq ($(WITH_DOCS),yes)
  set -e; for d in ${DOCDIRS}; do $(MAKE) -C $${d} install; done