mark small_vector get_allocator() as const

Summary: This function should be marked const.

Reviewed By: yfeldblum

Differential Revision: D16793980

fbshipit-source-id: 3e98f999c3e00caa0d238fa29c082ab6e819bf2a

folly/small_vector.h

@@ -563,7 +563,7 @@ class small_vector : public detail::small_vector_base<
                                      : BaseType::policyMaxSize();
   }
 
-  allocator_type get_allocator() {
+  allocator_type get_allocator() const {
     return {};
   }