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 <johannes.schindelin@gmx.de>

compat/poll/poll.c

@@ -269,6 +269,20 @@ win32_compute_revents_socket (SOCKET h, int sought, long lNetworkEvents)
   return happened;
 }
 
+#include <windows.h>
+#include "compat/win32/lazyload.h"
+
+static ULONGLONG CompatGetTickCount64(void)
+{
+	DECLARE_PROC_ADDR(kernel32.dll, ULONGLONG, GetTickCount64, void);
+
+	if (!INIT_PROC_ADDR(GetTickCount64))
+		return (ULONGLONG)GetTickCount();
+
+	return GetTickCount64();
+}
+#define GetTickCount64 CompatGetTickCount64
+
 #else /* !MinGW */
 
 /* Convert select(2) returned fd_sets into poll(2) revents values.  */