8355498: [AIX] Adapt code for C++ VLA rule

Backport-of: 2447b9812a9f7316a2313f70db4974534fceb9d9

Author: Suchismith Roy

src/hotspot/os/aix/os_perf_aix.cpp

@@ -73,7 +73,7 @@ enum {
  * Get info for requested PID from /proc/<pid>/psinfo file
  */
 static bool read_psinfo(const u_longlong_t& pid, psinfo_t& psinfo) {
-  static size_t BUF_LENGTH = 32 + sizeof(u_longlong_t);
+  const size_t BUF_LENGTH = 32 + sizeof(u_longlong_t);
 
   FILE* fp;
   char buf[BUF_LENGTH];