mail-client/thunderbird: revbump to propagate libpng changes

Needed for --changed-use users to pick up the change.

Bug: https://bugs.gentoo.org/824826
See: 16f32b5962
Signed-off-by: Sam James <sam@gentoo.org>

mail-client/thunderbird/metadata.xml

@@ -28,9 +28,9 @@
 		instead of bundled.</flag>
 	<flag name="system-libvpx">Use the system-wide <pkg>media-libs/libvpx</pkg>
 		instead of bundled.</flag>
+	<flag name="system-png">Use the system-wide <pkg>media-libs/libpng</pkg>
+		instead of bundled (requires APNG patches)</flag>
 	<flag name="system-webp">Use the system-wide <pkg>media-libs/libwebp</pkg>
 		instead of bundled.</flag>
-
-
 </use>
 </pkgmetadata>