win32: include "windows.h" not "Windows.h"

Supports handling case-sensitive filesystems (possibly when
cross-compiling).

Author: ethomson

adopt.c

@@ -15,7 +15,7 @@
 #include "adopt.h"
 
 #ifdef _WIN32
-# include <Windows.h>
+# include <windows.h>
 #else
 # include <fcntl.h>
 # include <sys/ioctl.h>