Do not print to console for which command

The command which would print warnings on some
platform. Redirect both stdout and stderr to
/dev/null while running the command.

Signed-off-by: Jiadong Zhu <Jiadong.Zhu@amd.com>
Change-Id: Ibc377681a31a14a3e306ab4fcb14d8d0c853fa86


[ROCm/rocminfo commit: d1efacb47f]

projects/rocminfo/rocminfo.cc

@@ -248,7 +248,7 @@ pair<string, int> exec(const char* cmd) {
 }
 
 static void DetectWSLEnvironment() {
-  auto process_ret = exec("which wslinfo");
+  auto process_ret = exec("which wslinfo > /dev/null 2>&1");
   if (process_ret.second)
     return;