VImage::get_typeof() returns GType

it was int, causing some compiler warnings on some platforms.

see https://github.com/jcupitt/libvips/issues/362

ChangeLog

@@ -4,6 +4,7 @@
 - small doc improvements
 - small vips7 C++ improvement
 - remove exception specifications from vips8 C++ interface
+- VImage::get_typeof() now returns GType
 
 7/10/15 started 8.2.0
 - added im_bufmagick2vips(), a vips7 wrapper for magick load from buffer

cplusplus/include/vips/VImage8.h

@@ -357,7 +357,7 @@ public:
 			free_fn, data, length ); 
 	}
 
-	int 
+	GType 
 	get_typeof( const char *field )
 	{
 		return( vips_image_get_typeof( this->get_image(), field ) );