fix issue when compile windows version

[witwall]

fix issue: '_mkdir' was not declared in this scope

pdf2htmlEX-0.13.6/src/util/path.cc: In function 'void pdf2htmlEX::create_directories(const string&)':
pdf2htmlEX-0.13.6/src/util/mingw.h:17:29: error: '_mkdir' was not declared in this scope
 #define mkdir(A, B) _mkdir(A)

[coolwanglu]

Thanks!

[coolwanglu]

Oh, just found that this is for the master branch, which should be incoming.
I'll merge it this time though.