include/plan9: define size_t to fix build breakage

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/10760043

include/plan9/libc.h

@@ -22,3 +22,7 @@ void	flagfn0(char*, char*, void(*fn)(void));
 void	flagfn1(char*, char*, void(*fn)(char*));
 void	flagfn2(char*, char*, void(*fn)(char*, char*));
 void	flagprint(int);
+
+// The libraries use size_t to avoid -Wconversion warnings from GCC
+// when calling standard library functions like memcpy.
+typedef unsigned long size_t;