Explicitly include limits.h instead of transitively assuming it

Recent upstream releases of LuaJIT have broken building this Lua Rock
entirely. PUC Lua and LuaJIT builds up to a certain point included this
header in lua.h, which meant that it was always around as a transitive
dependency. Recent upstream LuaJIT builds no longer include the header
since they aren't using it directly any more, but that means things that
include lua.h don't get it for free any more. It should never have been
assumed anyway, but that's been the status quo for a while.

This fixes build errors when using LuaJIT headers instead of PUC Lua:

```
lutf8lib.c: In function ‘Lutf8_codepoint’:
lutf8lib.c:398:22: error: ‘INT_MAX’ undeclared (first use in this function)
  398 |   if (pose - posi >= INT_MAX)  /* (lua_Integer -> int) overflow? */
      |                      ^~~~~~~
lutf8lib.c:12:1: note: ‘INT_MAX’ is defined in header ‘<limits.h>’; did you forget to ‘#include <limits.h>’?
   11 | #include "unidata.h"
  +++ |+#include <limits.h>
   12 |
lutf8lib.c:398:22: note: each undeclared identifier is reported only once for each function it appears in
  398 |   if (pose - posi >= INT_MAX)  /* (lua_Integer -> int) overflow? */
      |                      ^~~~~~~
```

This is because `INT_MAX` is defined appropriately for a given platfrom
in in limits.h.

lutf8lib.c

@@ -7,6 +7,7 @@
 #include <assert.h>
 #include <string.h>
 #include <stdint.h>
+#include <limits.h>
 #include <stdlib.h>
 
 #include "unidata.h"