Fix compiler error on some GHC versions

Fixes the following error:

    src/ShellCheck/CFGAnalysis.hs:1394:40: error:
	* Couldn't match expected type `[S.Set a]'
		      with actual type `M.Map String FunctionValue'
	* In the second argument of `($)', namely
	    `mapStorage $ sFunctionTargets state'
	  In the expression: S.unions $ mapStorage $ sFunctionTargets state
	  In an equation for `declaredFuncs':
	      declaredFuncs = S.unions $ mapStorage $ sFunctionTargets state
	* Relevant bindings include
	    declaredFuncs :: S.Set a
	      (bound at src/ShellCheck/CFGAnalysis.hs:1394:13)

src/ShellCheck/CFGAnalysis.hs

@@ -1391,7 +1391,7 @@ analyzeControlFlow params t =
     getFunctionTargets :: InternalState -> M.Map Node FunctionDefinition
     getFunctionTargets state =
         let
-            declaredFuncs = S.unions $ mapStorage $ sFunctionTargets state
+            declaredFuncs = S.unions $ M.elems $ mapStorage $ sFunctionTargets state
             getFunc d =
                 case d of
                     FunctionDefinition _ entry _ -> Just (entry, d)