Fix building on FreeBSD

Summary:
FreeBSD uses jemalloc as the base malloc implementation.
The patch has been functional on FreeBSD as of the MariaDB 10.2 port.
Closes https://github.com/facebook/rocksdb/pull/3386

Differential Revision: D6765742

Pulled By: yiwu-arbug

fbshipit-source-id: d55dbc082eecf640ef3df9a21f26064ebe6587e8

db/malloc_stats.cc

@@ -16,8 +16,13 @@
 namespace rocksdb {
 
 #ifdef ROCKSDB_JEMALLOC
+#ifdef __FreeBSD__
+#include <malloc_np.h>
+#define je_malloc_stats_print malloc_stats_print
+#else
 #include "jemalloc/jemalloc.h"
-
+#endif
+  
 typedef struct {
   char* cur;
   char* end;