Fix compilation with gcc 4.4 under CentOS 6

Fix

	'friend' keyword needs 'class'

error introduced by 035c29e6a2 (Make wxRescaleCoord() safer by requiring
explicitly using From/To, 2021-07-13) which broke the build with gcc
4.4.7 used under CentOS 6.

Closes https://github.com/wxWidgets/wxWidgets/pull/2450

include/wx/private/rescale.h

@@ -70,7 +70,7 @@ private:
     const wxSize m_oldScale;
 
     // Only it can create objects of this class.
-    friend wxRescaleCoordWithValue<T>;
+    friend class wxRescaleCoordWithValue<T>;
 };
 
 // Specialization for just a single value.
@@ -95,7 +95,7 @@ private:
     const wxSize m_oldScale;
 
     // Only it can create objects of this class.
-    friend wxRescaleCoordWithValue<int>;
+    friend class wxRescaleCoordWithValue<int>;
 };
 
 template <typename T>