tor-0.2.5.4-alpha and git/HEAD fail to build when --disable-buf-freelists

When tor is configured with --disable-buf-freelists, and RUNNING_DOXYGEN is not defined, buffers.c fails to compile because total_bytes_allocated_in_chunks is defined in src/or/buffers.c within an #if ... #else block and yet used in both the #if ... #else and #else .. #endif block. By moving its declaration before the #if ... #else ... #endif, we expand its visibility to both blocks where it is used.