Fix crash when using rocksdb and jemalloc but scope it to just jemalloc

Former-commit-id: 6fa0cfd35fd1158f190db27bfb734ea2f763cb20

src/new.cpp

@@ -32,3 +32,10 @@ void operator delete(void *p, std::size_t) noexcept
 }
 
 #endif
+
+#if defined(USE_JEMALLOC)
+extern "C" size_t malloc_usable_size(void *ptr)
+{
+    return zmalloc_usable(ptr);
+}
+#endif