Build all bionic test files -Werror and fix our one warning.

Change-Id: I62bb0528fd1bcb9aa4c61e44c78158a592690fc7
diff --git a/tests/dlfcn_test.cpp b/tests/dlfcn_test.cpp
index 18963cf..8b3f11c 100644
--- a/tests/dlfcn_test.cpp
+++ b/tests/dlfcn_test.cpp
@@ -61,7 +61,7 @@
   uint32_t (*f)(void);
   f = reinterpret_cast<uint32_t (*)(void)>(dlsym(handle, "dlsym_local_symbol_get_taxicab_number_using_dlsym"));
   ASSERT_TRUE(f != NULL);
-  ASSERT_EQ(1729, f());
+  ASSERT_EQ(1729U, f());
 }
 
 TEST(dlfcn, dlopen_noload) {