Redirect system_profiler stderr to /dev/null

System profiler sends useless output to stderr in normal circumstances. E.g. when requesting the SPDisplaysDataType, it's very common to get the following to stderr:

2013-05-10 17:44:33.297 system_profiler[47076:707] Error -536870184 (e00002d8) occurred reading HDCP register data in System Profiler.