fix erroneous feature test macro check for ppoll

commit f247462b0831cbf163e976ee9a909748c674b88b incorrectly hid ppoll
in the presence of _GNU_SOURCE due to an oversight that defining
_BSD_SOURCE does not implicitly define _GNU_SOURCE. at present,
headers still have to explicitly check for each feature profile level;
this may be changed at some point in the future via features.h, but
has not been changed yet.

include/poll.h

@@ -36,7 +36,7 @@ struct pollfd {
 
 int poll (struct pollfd *, nfds_t, int);
 
-#ifdef _BSD_SOURCE
+#if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 #define __NEED_time_t
 #define __NEED_struct_timespec
 #define __NEED_sigset_t
@@ -45,7 +45,7 @@ int ppoll(struct pollfd *, nfds_t, const struct timespec *, const sigset_t *);
 #endif
 
 #if _REDIR_TIME64
-#ifdef _BSD_SOURCE
+#if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 __REDIR(ppoll, __ppoll_time64);
 #endif
 #endif