Correct Windows include file name case, simplifying cross-compilation
Correct Windows include file name case, simplifying cross-compilation

When cross compiling to Windows, libssh2.h include Windows header files
with upper case filenames : BaseTsd.h and WinSock2.h.

These files have lowercase names with mingw-w64 (iirc, it's the same with
mingw). And as on Windows, being lowercase or uppercase does not matter.

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2