Switch to fork of backwardcpp with fixed line format

CMakeLists.txt

@@ -7,7 +7,7 @@ find_package(MPI REQUIRED)
 include(FetchContent)
 
 FetchContent_Declare(backwardcpp
-    GIT_REPOSITORY https://github.com/bombela/backward-cpp
+    GIT_REPOSITORY https://github.com/stelzch/backward-cpp
     GIT_TAG master
 )
 FetchContent_MakeAvailable(backwardcpp)