cmd/gc: fix windows build

c:\src\go\pkg\obj\windows_amd64\libgc.a(lex.o): In function `catcher':
c:/src/go/src/cmd/gc/lex.c:181: undefined reference to `noted'

LGTM=0intro
R=0intro
CC=golang-codereviews
https://golang.org/cl/63270043

src/cmd/gc/lex.c

@@ -164,6 +164,7 @@ fault(int s)
 	fatal("fault");
 }
 
+#ifdef	PLAN9
 void
 catcher(void *v, char *s)
 {
@@ -180,6 +181,7 @@ catcher(void *v, char *s)
 	}
 	noted(NDFLT);
 }
+#endif
 
 void
 doversion(void)