Rationalize visibility.

If it's C or POSIX, it's in. If it's BSD or GNU, it's guarded by __USE_BSD
or __USE_GNU.

Bug: https://code.google.com/p/android/issues/detail?id=194631
Change-Id: Ife51a21c2b37b060db56780d29c929805b199cb6
diff --git a/libc/include/pwd.h b/libc/include/pwd.h
index e881129..0b39a78 100644
--- a/libc/include/pwd.h
+++ b/libc/include/pwd.h
@@ -105,7 +105,7 @@
 #ifdef __LP64__
   char* pw_gecos;
 #else
-  // Note: On LP32, we define pw_gecos to pw_passwd since they're both NULL.
+  /* Note: On LP32, we define pw_gecos to pw_passwd since they're both NULL. */
 # define pw_gecos pw_passwd
 #endif
   char* pw_dir;