Fixed FALLOC_FL_KEEP_SIZE undefined (#5614)

Summary:
Fix `error: ‘FALLOC_FL_KEEP_SIZE’` undeclared error in `io_posix.cc` during Vagrant build in CentOS as per issue https://github.com/facebook/rocksdb/issues/5599
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5614

Differential Revision: D17217960

fbshipit-source-id: ef736c51b16833107fd9ccc7917ed1def2a8d02c

env/io_posix.cc

@@ -14,8 +14,10 @@
 #include <algorithm>
 #if defined(OS_LINUX)
 #include <linux/fs.h>
+#ifndef FALLOC_FL_KEEP_SIZE
 #include <linux/falloc.h>
 #endif
+#endif
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>