Updated submodules to the new repository URLs

.gitmodules

 [submodule "m4"]
 	path = m4
-	url = https://git.xiph.org/icecast/m4.git
+	url = https://git.xiph.org/icecast-m4.git
 [submodule "src/common"]
 	path = src/common
-	url = https://git.xiph.org/icecast/common.git
+	url = https://git.xiph.org/icecast-common.git