Fix build failure when USE_GSTREAMER_GL=off.  See
<https://bugs.webkit.org/show_bug.cgi?id=196440>.

This patch is taken from the upstream source repository:
<https://git.webkit.org/?p=WebKit.git;a=commitdiff;h=e2dd6decbe25ea9498f1ba213808f34b232740c7>.

diff --git a/Source/WebCore/platform/graphics/gstreamer/MediaPlayerPrivateGStreamerBase.cpp b/Source/WebCore/platform/graphics/gstreamer/MediaPlayerPrivateGStreamerBase.cpp
index 00a2af6489e..5cb5f7536ac 100644
--- a/Source/WebCore/platform/graphics/gstreamer/MediaPlayerPrivateGStreamerBase.cpp
+++ b/Source/WebCore/platform/graphics/gstreamer/MediaPlayerPrivateGStreamerBase.cpp
@@ -1000,11 +1000,13 @@ void MediaPlayerPrivateGStreamerBase::updateTextureMapperFlags()
         break;
     }
 
+#if USE(GSTREAMER_GL)
     // When the imxvpudecoder is used, the texture sampling of the
     // directviv-uploaded texture returns an RGB value, so there's no need to
     // convert it.
     if (m_videoDecoderPlatform != WebKitGstVideoDecoderPlatform::ImxVPU)
         m_textureMapperFlags |= TEXTURE_MAPPER_COLOR_CONVERT_FLAG;
+#endif
 }
 #endif