[mypyc] fix build on windows (python#11316)

Trying to fix mypyc/mypy_mypyc-wheels#28,
not sure why other windows builds don't complain...

```
build\misc_ops.c(515): error C2065: 'ssize_t': undeclared identifier
build\misc_ops.c(515): error C2146: syntax error: missing ';' before identifier 'n'
build\misc_ops.c(515): error C2065: 'n': undeclared identifier
build\misc_ops.c(517): error C2065: 'n': undeclared identifier
build\misc_ops.c(520): error C2065: 'n': undeclared identifier
build\misc_ops.c(521): error C2065: 'n': undeclared identifier
```

Co-authored-by: hauntsaninja <>

mypyc/lib-rt/misc_ops.c

@@ -512,7 +512,7 @@ int CPySequence_CheckUnpackCount(PyObject *sequence, Py_ssize_t expected) {
 
 // Parse an integer (size_t) encoded as a variable-length binary sequence.
 static const char *parse_int(const char *s, size_t *len) {
-    ssize_t n = 0;
+    Py_ssize_t n = 0;
     while ((unsigned char)*s >= 0x80) {
         n = (n << 7) + (*s & 0x7f);
         s++;