Use begin/end macros in cogl-primitives.h

Protect the people using a C++ compiler from the dark abyss of C code.

http://bugzilla.openedhand.com/show_bug.cgi?id=2049

cogl/cogl-primitives.h

@@ -24,6 +24,10 @@
 #ifndef __COGL_PRIMITIVES_H
 #define __COGL_PRIMITIVES_H
 
+#include <glib.h>
+
+G_BEGIN_DECLS
+
 /**
  * SECTION:cogl-primitives
  * @short_description: Functions that draw various primitive 3D shapes
@@ -181,4 +185,6 @@ cogl_polygon (const CoglTextureVertex  *vertices,
               unsigned int              n_vertices,
               gboolean                  use_color);
 
+G_END_DECLS
+
 #endif /* __COGL_PRIMITIVES_H */