lib9: fix Solaris build

In file included from src/lib9/utf/utfecpy.c:17:0:
src/lib9/utf/utfdef.h:28:0: error: "nil" redefined [-Werror]
In file included from src/lib9/utf/utfrrune.c:17:0:
src/lib9/utf/utfdef.h:28:0: error: "nil" redefined [-Werror]

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/63410043

src/lib9/utf/utfdef.h

@@ -25,4 +25,3 @@ typedef unsigned int		uint;
 typedef unsigned long		ulong;
 
 #define nelem(x) (sizeof(x)/sizeof((x)[0]))
-#define nil ((void*)0)