LibC: Define ptrdiff_t limits

Userland/Libraries/LibC/bits/stdint.h

@@ -123,4 +123,7 @@ typedef __INTMAX_TYPE__ intmax_t;
 
 #define SIZE_MAX ((size_t)-1)
 
+#define PTRDIFF_MAX __PTRDIFF_MAX__
+#define PTRDIFF_MIN (-__PTRDIFF_MAX__ - 1)
+
 __END_DECLS