build: define ssize_t for MSVC in ghostty.h

MSVC's <sys/types.h> does not define ssize_t (it is a POSIX type).
This causes the translate-c build step to fail when translating
ghostty.h on Windows. Use SSIZE_T from <BaseTsd.h> which is the
Windows SDK equivalent.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

include/ghostty.h

@@ -17,6 +17,11 @@ extern "C" {
 #include <stdint.h>
 #include <sys/types.h>
 
+#ifdef _MSC_VER
+#include <BaseTsd.h>
+typedef SSIZE_T ssize_t;
+#endif
+
 //-------------------------------------------------------------------
 // Macros