Address additional warnings found by GCC 4.9.

Change-Id: I8aae6c3e1ccc2294f4db455e1b4f3b4c34a00aaf
diff --git a/liblog/tests/liblog_test.cpp b/liblog/tests/liblog_test.cpp
index ec35e45..393e2cd 100644
--- a/liblog/tests/liblog_test.cpp
+++ b/liblog/tests/liblog_test.cpp
@@ -544,7 +544,7 @@
     EXPECT_LE(LOGGER_ENTRY_MAX_PAYLOAD - sizeof(big_payload_tag),
               static_cast<size_t>(max_len));
 
-    EXPECT_EQ(ret, max_len + sizeof(big_payload_tag));
+    EXPECT_EQ(ret, max_len + static_cast<ssize_t>(sizeof(big_payload_tag)));
 }
 
 TEST(liblog, dual_reader) {