ASACORE-1153 Fix data type in file transfer sample

modified:   alljoyn_core/samples/FileTransfer/FileTransferService.cc
bufferLength defined as std::streamsize, would cause bug in vc++2010 express on win7
so it should be defined as size_t type.

Change-Id: I7737ae702267b0636f8f5cb750360cb3d2d15040
Signed-off-by: europelee <wwweurope@163.com>
Merge "ASACORE-1153 Fix data type in file transfer sample"