rfs() doens't work inside calc()

I'm using RFS v9.0.3 with PostCSS. The `rfs()` function doesn't work when wrapped in a `calc()` function.


**Input:**
```css
margin-right: calc(rfs(2rem) * 2);
```

**Output:**
```css
margin-right: rfs(2rem) * 2;
```

**Expected Output:**
```css
margin-right: calc(calc(1.325rem + 0.9vw) * 2);
```