Fixed compile warning in VS2013

This pull request fixes a minor warning in VS2013, which caused the build/VS2010/flatc.vcxproj project to fail building (warnings treated as errors).

The warning message was: 

>Warning	2	warning C4244: 'argument' : conversion from 'int' to 'char', possible loss of data	 src\idl_parser.cpp	111	1	flatc