Fixed compiler error in visual studio 7.1

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402904

dlib/matrix/matrix_expressions.h

@@ -1610,7 +1610,7 @@ namespace dlib
         T operator() (
             long,
             long c
-        ) const { return std::pow(10,start + c*inc);  }
+        ) const { return std::pow((T)10,start + c*inc);  }
 
         T operator() (
             long c