profile_select: Return int instead of u32 for GetIndex()

Qt uses a signed value to represent indices. We should follow this
convention where applicable to avoid unnecessary sign-conversion
warnings, as well as making it easier to interoperate with other aspects
of Qt.

While we're at it, we can also make a sign-conversion explicit.

src/yuzu/applets/profile_select.cpp

@@ -136,7 +136,7 @@ bool QtProfileSelectionDialog::GetStatus() const {
     return ok;
 }
 
-u32 QtProfileSelectionDialog::GetIndex() const {
+int QtProfileSelectionDialog::GetIndex() const {
     return user_index;
 }