Add the empty simpleType into XSDs

[onacit]

It seems these two XSDs suffered from the missing empty type.

diff --git a/jingle-apps-rtp-rtcp-fb.xsd b/jingle-apps-rtp-rtcp-fb.xsd
index 6682a10..b46a2c2 100644
--- a/jingle-apps-rtp-rtcp-fb.xsd
+++ b/jingle-apps-rtp-rtcp-fb.xsd
@@ -43,4 +43,10 @@
     </xs:simpleContent>
   </xs:complexType>
 
+  <xs:simpleType name='empty'>
+    <xs:restriction base='xs:string'>
+      <xs:enumeration value=''/>
+    </xs:restriction>
+  </xs:simpleType>
+
 </xs:schema>
diff --git a/jingle-apps-rtp-rtp-hdrext.xsd b/jingle-apps-rtp-rtp-hdrext.xsd
index 9b7a761..53c122f 100644
--- a/jingle-apps-rtp-rtp-hdrext.xsd
+++ b/jingle-apps-rtp-rtp-hdrext.xsd
@@ -45,4 +45,10 @@
     </xs:simpleContent>
   </xs:complexType>
 
+  <xs:simpleType name='empty'>
+    <xs:restriction base='xs:string'>
+      <xs:enumeration value=''/>
+    </xs:restriction>
+  </xs:simpleType>
+
 </xs:schema>