Prevents the VideoToolbox encoder to be added to the ms_voip_filter_descs list at compilation time

That filter must be added at runtime when the VideoToolbox API is present on the system

src/videofilters/videotoolbox.c

@@ -406,8 +406,6 @@ MSFilterDesc ms_vt_h264_enc = {
 	.methods = h264_enc_methods
 };
 
-MS_FILTER_DESC_EXPORT(ms_vt_h264_enc)
-
 
 
 #define H264_NALU_HEAD_SIZE 4
@@ -796,6 +794,9 @@ void _register_videotoolbox_if_supported(MSFactory *factory) {
 		
 		ms_factory_register_filter(factory, &ms_vt_h264_enc);
 		ms_factory_register_filter(factory, &ms_vt_h264_dec);
+	} else {
+		ms_warning("Cannot register VideoToolbox filters. Those filters"
+			" require iOS 8 or MacOSX 10.8");
 	}
 }