use Z3_fallthrough instead of __falthrough directly to avoid messing with reserved identifiers

Signed-off-by: Nuno Lopes <nlopes@microsoft.com>

src/ast/ast.cpp

@@ -516,7 +516,7 @@ inline unsigned ast_array_hash(T * const * array, unsigned size, unsigned init_v
         switch (size) {
         case 2:
             b += array[1]->hash();
-            __fallthrough;
+            Z3_fallthrough;
         case 1:
             c += array[0]->hash();
         }