Resolve davisking/dlib#2463 (#2492)

* Resolve davisking/dlib#2463

Fixes missing ssize_t definition on Windows

* Define ssize_t only on Windows 64-bit

dlib/external/pybind11/include/pybind11/numpy.h

@@ -26,6 +26,10 @@
 #if defined(_MSC_VER)
 #  pragma warning(push)
 #  pragma warning(disable: 4127) // warning C4127: Conditional expression is constant
+#ifdef _WIN64
+#include <BaseTsd.h>
+using ssize_t = SSIZE_T;
+#endif
 #endif
 
 /* This will be true on all flat address space platforms and allows us to reduce the