Fix QgsComposerMap sip/cpp build error

Author: dakcarto

src/core/composer/qgscomposermap.cpp

@@ -1794,7 +1794,7 @@ void QgsComposerMap::setGridBlendMode( QPainter::CompositionMode blendMode )
   g->setBlendMode( blendMode );
 }
 
-QPainter::CompositionMode QgsComposerMap::gridBlendMode()
+QPainter::CompositionMode QgsComposerMap::gridBlendMode() const
 {
   const QgsComposerMapGrid* g = constFirstMapGrid();
   return g->blendMode();

src/core/composer/qgscomposermap.h

@@ -432,7 +432,7 @@ class CORE_EXPORT QgsComposerMap : public QgsComposerItem
     QgsLineSymbolV2* gridLineSymbol();
 
     /** Returns the grid's blending mode */
-    QPainter::CompositionMode gridBlendMode();
+    QPainter::CompositionMode gridBlendMode() const;
     /** Sets the grid's blending mode*/
     void setGridBlendMode( QPainter::CompositionMode blendMode );