poll: lazy-load GetTickCount64()

This fixes the compilation, actually, as we still did not make the jump to
post-Windows XP completely: we still compile with _WIN32_WINNT set to
0x0502 (which corresponds to Windows Server 2003 and is technically
greater than Windows XP's 0x0501).

However, GetTickCount64() is only available starting with Windows
Vista/Windows Server 2008.

Let's just lazy-load the function, which should also help Git for Windows
contributors who want to reinstate Windows XP support.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho