hide the inevitable SystemProfiler warning

for some reason, this line shows up on stderr anytime I (or the
find-device-port script) run `system_profiler SPUSBDataType`

2020-02-24 23:19:14.656 system_profiler[23932:4118769] SPUSBDevice:
IOCreatePlugInInterfaceForService failed 0xe00002be

I got annoyed enough to suppress it, so I could see the other output
that I actually care about.

bin/find-device-port-macos

@@ -12,7 +12,7 @@ my $pid = shift @ARGV;
 
 # ioreg might be more machine-readable than system_profiler, but I haven't been able to
 # get it to produce useful output
-my @output = qx(/usr/sbin/system_profiler SPUSBDataType);
+my @output = qx(/usr/sbin/system_profiler SPUSBDataType 2> /dev/null);
 
 my $serial   = "";
 my $location = "";