Bug 576776 - Webkit extension missing from org.eclipse.swt.gtk.linux.aarch64

Fixed org.eclipse.swt.gtk.linux.aarch64/build.properties to include webkitextensions

Change-Id: I420e499c1e2da2fccf4f48971341a95dc1f886b6
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.swt.binaries/+/187595
Tested-by: Alexander Kurtakov <akurtako@redhat.com>
Reviewed-by: Alexander Kurtakov <akurtako@redhat.com>