make bounded() unambiguous

```
std::size_t is the unsigned integer type of the result of the sizeof
operator
```

this can make the bounded call ambiguous as it either sports
an int(int,int) signature or an quint32(quint32,quint32) signature. with
size_t being unsigned and 0 being int it's not necessarily clear to the
compiler which overload to use, so make it clear by using usigned
everywhere.
this specifically fixes arm32 compatibility on ubuntu 20.04

kcms/users/src/user.cpp

@@ -179,7 +179,8 @@ saltCharacter() {
                        "abcdefghijklmnopqrstuvxyz"
                        "./0123456789";
 
-    int index = QRandomGenerator::system()->bounded(0, (sizeof(saltCharacters)/sizeof(*saltCharacters)));
+    const quint32 index =
+        QRandomGenerator::system()->bounded(0u, (sizeof(saltCharacters)/sizeof(*saltCharacters)));
 
     return saltCharacters[index];
 }