plistutil: fix missing ssize_t definition when compiling with MSVC

tools/plistutil.c

diff --git a/tools/plistutil.c b/tools/plistutil.c
index 37bb044..e2ea286 100644
--- a/tools/plistutil.c
+++ b/tools/plistutil.c
@@ -41,6 +41,9 @@
 #pragma warning(disable:4996)
 #define STDIN_FILENO _fileno(stdin)
 #define strtok_r strtok_s
+
+#include <basetsd.h>
+typedef SSIZE_T ssize_t;
 #endif
 
 typedef struct _options