JEMALLOC_USABLE_SIZE_CONST is not defined in FreeBSD's jemalloc(3), define the
macro to "const".

This prevents build failures in FreeBSD.

Reference upstream bug report: https://jira.mariadb.org/browse/MDEV-20248
--- storage/rocksdb/rocksdb/port/jemalloc_helper.h.orig	2019-08-12 02:20:55 UTC
+++ storage/rocksdb/rocksdb/port/jemalloc_helper.h
@@ -12,6 +12,10 @@
 #include <jemalloc/jemalloc.h>
 #endif
 
+#ifdef __FreeBSD__
+#define JEMALLOC_USABLE_SIZE_CONST const
+#endif
+
 #ifndef JEMALLOC_CXX_THROW
 #define JEMALLOC_CXX_THROW
 #endif