fix(NumberTheory/MahlerMeasure): add missing public section (#32727)

We add the line 
```
@[expose] public section
```
which was missing when the file was created in #31732.

Co-authored-by: fbarroero <fbarroero@gmail.com>

Author: fbarroero

Mathlib/NumberTheory/MahlerMeasure.lean

@@ -21,6 +21,8 @@ The main purpose of this file is to prove Northcott's Theorem for the Mahler mea
   of degree at most `n` and Mahler measure at most `B`.
 -/
 
+@[expose] public section
+
 namespace Polynomial
 
 open Int