Use lower case file name with #include

I tried to cross compile harfbuzz for Windows and an error was generated because `DWrite_1.h` was not found.
This happened because the filesystem is case sensitive and for this reason that include file was not found.
The right name of the file to be used is `dwrite_1.h`, with all letters not capitalized:
https://docs.microsoft.com/en-us/windows/desktop/api/dwrite_1/
I also verified in the installation of VS2017 with Windows Kit v10 and in that place it was also lower case.
So, in my opinion it should be better to change this.

src/hb-directwrite.cc

@@ -28,7 +28,7 @@
 
 #include "hb-shaper-impl.hh"
 
-#include <DWrite_1.h>
+#include <dwrite_1.h>
 
 #include "hb-directwrite.h"