Visual Studio 2012 build fix.

Without this, I get this error:

Error	1	error C3848: expression having type 'const std::_Bind<_Forced,_Ret,_Fun,_V0_t,_V1_t,_V2_t,_V3_t,_V4_t,_V5_t,<unnamed-symbol>>' would lose some const-volatile qualifiers in order to call 'void std::_Bind<_Forced,_Ret,_Fun,_V0_t,_V1_t,_V2_t,_V3_t,_V4_t,_V5_t,<unnamed-symbol>>::operator ()(void)'	native\thread\thread.h	245	1	native

Does it really need to be const?