Define ssize_t for MSVC

ssize_t is not available in visual studio, nor is sys/param.h We typedef
intmax_t to ssize_t

lib/capn.c

@@ -11,7 +11,9 @@
 
 #include <stdlib.h>
 #include <string.h>
+#ifndef _MSC_VER
 #include <sys/param.h>
+#endif
 
 #define STRUCT_PTR 0
 #define LIST_PTR 1

lib/capnp_c.h

@@ -14,6 +14,11 @@
 #include <stdint.h>
 #include <stdio.h>
 
+// ssize_t is not defined in stdint.h in MSVC. 
+#ifdef _MSC_VER
+typedef intmax_t ssize_t;
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif