Fix casing of "BaseTsd.h" include

Microsoft isn't consistent about the casing of header filenames in the
Windows SDK and documentation (e.g. "Windows.h" vs. "windows.h"), but
this doesn't matter on a case-insensitive filesystem.

If you are cross-compiling for Windows (e.g. using gcc), then the case
matters, and is practically controlled by the w32api headers provided by
MinGW64 project.

Author: jon-turney

win32/regex.h

@@ -12,7 +12,7 @@ extern "C" {
 #define CHARCLASS_NAME_MAX 14
 #define RE_DUP_MAX 255
 
-#include <BaseTsd.h>
+#include <basetsd.h>
 typedef SSIZE_T regoff_t;
 // #include <bits/alltypes.h>