lib9: fix memory leak in p9getwd()

        although the comment says it uses libc's getenv, without NOPLAN9DEFINES
        it actually uses p9getenv which strdups.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/6285046

src/lib9/getwd.c

@@ -26,10 +26,9 @@ THE SOFTWARE.
 #include <u.h>
 #include <errno.h>
 #include <sys/stat.h>
+#define NOPLAN9DEFINES
 #include <libc.h>
 
-#undef getwd
-
 char*
 p9getwd(char *s, int ns)
 {