Improve authorization_set test coverage.

Change-Id: I8dd1830db8c19be07cef768c63c9ecfa3e16ae21
diff --git a/key_blob_test.cpp b/key_blob_test.cpp
index 3f4c6e4..da2b96a 100644
--- a/key_blob_test.cpp
+++ b/key_blob_test.cpp
@@ -37,6 +37,7 @@
 
 namespace keymaster {
 
+bool operator==(const AuthorizationSet& a, const AuthorizationSet& b);
 bool operator==(const AuthorizationSet& a, const AuthorizationSet& b) {
     if (a.size() != b.size())
         return false;