Stop LCC from warning about null pointer conversion to function pointer

Fixed LCC to correctly diagnose expressions with NPC. It no longer reports messages such as

    warning: conversion from `pointer to void' to `pointer to void function(void)' is compiler dependent

code/tools/lcc/src/c.h

@@ -577,6 +577,7 @@ extern Tree cnsttree(Type, ...);
 extern Tree consttree(unsigned int, Type);
 extern Tree eqtree(int, Tree, Tree);
 extern int iscallb(Tree);
+extern int isnullptr(Tree);
 extern Tree shtree(int, Tree, Tree);
 extern void typeerror(int, Tree, Tree);