fix the mremap signature

The mremap definition was incorrect (unsigned long instead of int) and
it was missing the optional new_address parameter.

Change-Id: Ib9d0675aaa098c21617cedc9b2b8cf267be3aec4
diff --git a/tests/sys_mman_test.cpp b/tests/sys_mman_test.cpp
index b0e40fd..dffb646 100644
--- a/tests/sys_mman_test.cpp
+++ b/tests/sys_mman_test.cpp
@@ -215,3 +215,7 @@
 
   ASSERT_EQ(0, munmap(map, pagesize));
 }
+
+TEST(sys_mman, mremap) {
+  ASSERT_EQ(MAP_FAILED, mremap(nullptr, 0, 0, 0));
+}