Fix FreeBSD building (#10575)

Summary:
FreeBSD doesn't have `JEMALLOC_USABLE_SIZE_CONST` so we need to define
it.

This fixes MariaDB MDEV-20248.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10575

Reviewed By: jay-zhuang

Differential Revision: D39057665

Pulled By: ajkr

fbshipit-source-id: 3874779d12a1dd5036324947f6372e6ad57a7b08

port/jemalloc_helper.h

@@ -21,6 +21,7 @@
 #ifdef ROCKSDB_JEMALLOC
 #ifdef __FreeBSD__
 #include <malloc_np.h>
+#define JEMALLOC_USABLE_SIZE_CONST const
 #else
 #define JEMALLOC_MANGLE
 #include <jemalloc/jemalloc.h>