Marking HIDL version of gatekeeper as optional.

To avoid incompatibility errors marking the hidl version of
gatekeeper as optional.

Bug: 268342724
Test: VtsHalGatekeeperTargetTest
Change-Id: Id9546844998ce88ed597eaae0c05a119fb9322c9
diff --git a/compatibility_matrices/compatibility_matrix.6.xml b/compatibility_matrices/compatibility_matrix.6.xml
index e19d2dd..1c76ee6 100644
--- a/compatibility_matrices/compatibility_matrix.6.xml
+++ b/compatibility_matrices/compatibility_matrix.6.xml
@@ -215,7 +215,7 @@
             <instance>default</instance>
         </interface>
     </hal>
-    <hal format="hidl" optional="false">
+    <hal format="hidl" optional="true">
         <name>android.hardware.gatekeeper</name>
         <version>1.0</version>
         <interface>