Fix casing of msctf.h header

see https://learn.microsoft.com/en-us/windows/win32/api/msctf/

(ran into this cross compiling using a case-sensitive filesystem)

Author: tmm1

src/keyboard_win.cc

@@ -10,7 +10,7 @@
 
 #include "string_conversion.h"
 #include <windows.h>
-#include <Msctf.h>
+#include <msctf.h>
 #include <ime.h>
 
 namespace {