Fix compiler warning on signed-unsigned compare

Compiler was generating a warning for test case that was comparing a
constant integer to a size. Cast the constant integer as size_t.

Signed-off-by: Ben Tyner <ben.tyner@ibm.com>
Change-Id: I7c1c5c852a3cdf0f2f1c6908cc3647c90fa635c3
diff --git a/test/flyweight_test.cpp b/test/flyweight_test.cpp
index fb982e1..9ed9eb6 100644
--- a/test/flyweight_test.cpp
+++ b/test/flyweight_test.cpp
@@ -34,5 +34,6 @@
     ASSERT_NE(f1, f2); // Pointing to different objects
     ASSERT_EQ(f1, f3); // Pointing to the same object
 
-    ASSERT_EQ(2, foo_factory.size()); // Only two entries in the flyweight
+    ASSERT_EQ(size_t(2),
+              foo_factory.size()); // Only two entries in the flyweight
 }