## `pluralize()` returns wrong plural for words ending in "x" (e.g. "boxs" instead of "boxes")

The `pluralize()` function in `boltons.strutils` correctly handles words ending in `s`, `ch`, and `sh` by appending `-es`, but it misses words ending in `x`. This means common words like `box`, `fox`, `tax`, and `prefix` get an incorrect bare `s` appended instead.

```python
from boltons import strutils

print(strutils.pluralize('box'))    # prints 'boxs'   (expected 'boxes')
print(strutils.pluralize('fox'))    # prints 'foxs'   (expected 'foxes')
print(strutils.pluralize('tax'))    # prints 'taxs'   (expected 'taxes')
print(strutils.pluralize('prefix')) # prints 'prefixs' (expected 'prefixes')
```

The same issue applies to capitalized and uppercase variants — `pluralize('Box')` gives `'Boxs'` and `pluralize('FOX')` gives `'FOXS'`.

Note that irregular `-x` words like `ox` (which pluralizes to `oxen`) should remain unaffected since they are resolved from the irregular map before any suffix rule runs.

**Expected behavior:** Words ending in `x` should follow the same `-es` pluralization rule as words ending in `s`, `ch`, or `sh`.

**Actual behavior:** Words ending in `x` get a bare `s` appended, producing invalid plurals like `boxs`, `foxs`, `taxes` → `taxs`.

The bug is in the condition inside `pluralize()` that checks for these special endings — `x` is simply not included in the check.

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