Fix inherited state vars for BMC

test/libsolidity/smtCheckerTests/imports/private_vars.sol

@@ -0,0 +1,16 @@
+==== Source: ERC20.sol ====
+pragma experimental SMTChecker;
+contract ERC20 {
+    uint256 private a;
+    function f() internal virtual {
+        a = 2;
+    }
+}
+==== Source: Token.sol ====
+pragma experimental SMTChecker;
+import "ERC20.sol";
+contract Token is ERC20 {
+    constructor() {
+      f();
+    }
+}