[BUILD] make install should depend on haproxy not "all"

Reported by Cherife Li : just doing a "make install" fails because it
depends on "all" which is equivalent to "help" if no TARGET was specified.
Make it depend on "haproxy" instead.
diff --git a/Makefile b/Makefile
index f9696df..bf172f6 100644
--- a/Makefile
+++ b/Makefile
@@ -488,7 +488,7 @@
 		install -m 644 doc/$$x.txt $(DESTDIR)$(DOCDIR) ; \
 	done
 
-install-bin: all
+install-bin: haproxy
 	install -d $(DESTDIR)$(SBINDIR)
 	install haproxy $(DESTDIR)$(SBINDIR)