Fix /singularity/singularity-master/indra/plugins/filepicker/llfilepicker.cpp:762:61: warning: multi-character character constant

OSTypes are four byte sequences, therefore we should use 'JP2 ' instead of 'JP2'
Breaking pad: warning: 'OSStatus CarbonEventHandler(OpaqueEventHandlerCallRef*, OpaqueEventRef*, void*)' defined but not used