Add a direct dependency from guava-gwt to checker-qual.

The GWT sources (specifically, GwtSerializationDependencies) use @nullable directly, so we shouldn't rely on relying on it indirectly through guava-jre.

However, what actually prompted this is a strange behavior in Compile-Testing, which Truth uses. This CL should help, though it might not be a fully solution.
google/compile-testing#149

RELNOTES=n/a

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=198882337

guava-gwt/pom.xml

@@ -95,6 +95,10 @@
       <classifier>gwt</classifier>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.checkerframework</groupId>
+      <artifactId>checker-qual</artifactId>
+    </dependency>
   </dependencies>
   <build>
     <plugins>