Restore build on older compilers

Older compilers, e.g., GCC 4.4., do not understand the
"simple-type-specifier" syntax of C++11's "friend" declarations.
Typical responses are

   error: a class-key must be used when declaring a friend
   error: friend declaration does not name a class or function

Restore build on older compilers (e.g., GCC prior to 4.7) by
inserting the 'class' keyword.

opm/polymer/fullyimplicit/BlackoilPolymerModel.hpp

@@ -51,7 +51,7 @@ namespace Opm {
         typedef typename Base::WellState WellState;
         // The next line requires C++11 support available in g++ 4.7.
         // friend Base;
-        friend BlackoilModelBase<Grid, BlackoilPolymerModel<Grid> >;
+        friend class BlackoilModelBase<Grid, BlackoilPolymerModel<Grid> >;
 
         /// Construct the model. It will retain references to the
         /// arguments of this functions, and they are expected to