Update robtk submodule URI

The unauthenticated git protocol on port 9418 is no longer supported.
https://github.blog/2021-09-01-improving-git-protocol-security-github/

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "robtk"]
 	path = robtk
-	url = git://github.com/x42/robtk
+	url = https://github.com/x42/robtk.git

Makefile.git

@@ -15,6 +15,7 @@ submodule_check:
 	  git submodule status \
 	  | grep -q "^-" \
 	  && $(MAKE) submodules || true
+	@-test -d .git -a .gitmodules && git submodule sync
 	@-test -d .git -a .gitmodules && \
 	  git submodule status \
 	  | grep -q "^+" \

README.md

@@ -45,7 +45,7 @@ Compiling fil4 requires the LV2 SDK, jack-headers, gnu-make, a c++-compiler,
 libpango, libcairo and openGL (sometimes called: glu, glx, mesa).
 
 ```bash
-  git clone git://github.com/x42/fil4.lv2.git
+  git clone https://github.com/x42/fil4.lv2.git
   cd fil4.lv2
   make submodules
   make