unistd: Avoid PLT entries with _FORTIFY_SOURCE

The change is meant to avoid unwanted PLT entries for the read_chk,
getdomainname_chk and getlogin_r_chk routines when _FORTIFY_SOURCE is set.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

debug/getdomainname_chk.c

@@ -26,3 +26,4 @@ __getdomainname_chk (char *buf, size_t buflen, size_t nreal)
 
   return getdomainname (buf, buflen);
 }
+libc_hidden_def (__getdomainname_chk)

debug/read_chk.c

@@ -25,3 +25,4 @@ __read_chk (int fd, void *buf, size_t nbytes, size_t buflen)
 
   return __read (fd, buf, nbytes);
 }
+libc_hidden_def (__read_chk)