Add missing static memory allocation.

Author: jonsimantov

ump/integration_test/src/integration_test.cc

@@ -91,6 +91,8 @@ class FirebaseUmpTest : public FirebaseTest {
   firebase::ump::ConsentInfo* consent_info_;
 };
 
+firebase::App* FirebaseUmpTest::shared_app_ = nullptr;
+
 void FirebaseUmpTest::SetUpTestSuite() {
   LogDebug("Initialize Firebase App.");