getpathp.c: fix compiler warning

wcsnlen_s() result type is size_t.

PC/getpathp.c

@@ -135,7 +135,7 @@ exists(wchar_t *filename)
 static int
 ismodule(wchar_t *filename, int update_filename) /* Is module -- check for .pyc/.pyo too */
 {
-    int n;
+    size_t n;
 
     if (exists(filename))
         return 1;