gitmodules: update mavlink submodule URL to gartfeo repository

Author: arthur.gevorgyan

.gitmodules

@@ -6,7 +6,7 @@
 	url = https://github.com/google/benchmark.git
 [submodule "modules/mavlink"]
 	path = modules/mavlink
-	url = https://github.com/ArduPilot/mavlink
+	url = https://github.com/gartfeo/mavlink
 [submodule "gtest"]
 	path = modules/gtest
 	url = https://github.com/ArduPilot/googletest