Fix compilation of lstrlib.c for Lua 5.1.

The removal of lualib.h from compat-5.3.h causes an unwanted replacement
of luaopen_string to luaopen_string_XXX. Include lualib.h in lprefix.h
before the corresponding macro definition to counter that.

Author: siffiejoe

lprefix.h

@@ -99,6 +99,7 @@ static void compat53_rawseti (lua_State *L, int i, lua_Integer n) {
 #endif /* ltablib_c */
 
 #ifdef lstrlib_c
+#include <lualib.h>
 /* move the string library open function out of the way (we only take
  * the string packing functions)!
  */