missing #includes complaints by MSVC 2017

[Yuriy Skalko]

> Dear LyX developers,
> 
> I just tried to compile LyX (commit
> 2c1753a5265c59ebd11fc465cdb119ff589ad965) on windows 10 with MSVC 2017
> toolkit, and encountered compiling errors of the sort:
> 
> 
>    src\DocIterator.h(335): error C2039: 'min': is not a member of 'std
> 
> and another one about std::isspace.
> 
> It's easy to fix by including the missing include files. After the
> following patch it compiles fine:

Hello Panruo Wu,

This is due to recent include header cleanup in LyX codebase.

Thanks for testing LyX build with different compilers.

Yuriy