Fix build for hurd/thread-self.c for i386.

We need to include hurd.h for libc_hidden_proto (__hurd_thread_self),
introduced in b44c1e1252 ("hurd: Fix using interposable
hurd_thread_self")

This the error log:

In file included from <command-line>:
./../include/libc-symbols.h:472:33: error: '__EI___hurd_thread_self' aliased to undefined symbol '__GI___hurd_thread_self'
  472 |   extern thread __typeof (name) __EI_##name \
      |                                 ^~~~~
./../include/libc-symbols.h:468:3: note: in expansion of macro '__hidden_ver2'
  468 |   __hidden_ver2 (, local, internal, name)
      |   ^~~~~~~~~~~~~
./../include/libc-symbols.h:476:41: note: in expansion of macro '__hidden_ver1'
  476 | #  define hidden_def(name)              __hidden_ver1(__GI_##name, name, name);
      |                                         ^~~~~~~~~~~~~
./../include/libc-symbols.h:557:32: note: in expansion of macro 'hidden_def'
  557 | # define libc_hidden_def(name) hidden_def (name)
      |                                ^~~~~~~~~~
thread-self.c:27:1: note: in expansion of macro 'libc_hidden_def'
   27 | libc_hidden_def (__hurd_thread_self)
      | ^~~~~~~~~~~~~~~
Message-Id: <ZGr6wj2UOxg3F0qH@jupiter.tail36e24.ts.net>

hurd/thread-self.c

@@ -16,6 +16,7 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
+#include <hurd.h>
 #include <hurd/signal.h>
 
 thread_t