meson: gl: fix build on windows

https://bugzilla.gnome.org/show_bug.cgi?id=791772
diff --git a/gst-libs/gst/gl/meson.build b/gst-libs/gst/gl/meson.build
index 68af27f..c6e9e03 100644
--- a/gst-libs/gst/gl/meson.build
+++ b/gst-libs/gst/gl/meson.build
@@ -579,10 +579,13 @@
       'win32/win32_message_source.c',
       'win32/gstglwindow_win32.c',
       'win32/gstglwindow_win32.c',
+      'win32/gstglcontext_wgl.c',
     ]
     enabled_gl_winsys += 'win32'
     gl_winsys_deps += gdi_dep
     enabled_gl_platforms += 'wgl'
+    glconf.set('GST_GL_HAVE_WINDOW_WIN32', 1)
+    glconf.set('GST_GL_HAVE_PLATFORM_WGL', 1)
   endif
 endif