cgo: set cgo compiler flags in backend.go

the C import line is recommended to be on a separate line
this allows to use the comments in the lines preceeding it
to setup various cpp compiler flags

this also uses cgo comments to pass additional import dirs
for the cpp compiler, these are needed for gopls to work

gui/backend.go

@@ -3,10 +3,13 @@ package main
 /* a wrapper around bitmask that exposes status to a QtQml gui.
    Have a look at the pkg/backend module for further enlightment. */
 
-import (
-	"C"
-	"unsafe"
+// #cgo pkg-config: Qt6Core Qt6Gui Qt6Widgets Qt6Quick Qt6QuickControls2
+// #cgo CXXFLAGS: -I ..
+import "C"
+
+import "unsafe"
 
+import (
 	"0xacab.org/leap/bitmask-vpn/pkg/backend"
 )