Experimentally move libc_hidden_proto(time) to time.h.

Experimentally move libc_hidden_proto(time) to time.h.
The only code change observed is actually a case
where we were not doing that as needed:

 00000000 <__GI_tzset>:
-53                     push   %ebx
-e8 00 00 00 00         call   6 <__GI_tzset+0x6>
-5b                     pop    %ebx
-81 c3 03 00 00 00      add    $0x3,%ebx
-                       R_386_GOTPC     _GLOBAL_OFFSET_TABLE_
 6a 00                  push   $0x0
-e8 fc ff ff ff         call   10 <__GI_tzset+0x10>
-                       R_386_PLT32     time
+e8 fc ff ff ff         call   3 <__GI_tzset+0x3>
+                       R_386_PC32      __GI_time
 3d ff 4e 98 45         cmp    $0x45984eff,%eax
 0f 9e c0               setle  %al
 0f b6 c0               movzbl %al,%eax
 50                     push   %eax
-e8 fc ff ff ff         call   21 <__GI_tzset+0x21>
+e8 fc ff ff ff         call   14 <__GI_tzset+0x14>
                        R_386_PC32      _time_tzset
 58                     pop    %eax
 5a                     pop    %edx
-5b                     pop    %ebx
 c3                     ret

No mass migration of libc_hidden_proto(foo) planned.
Lets wait for potential fallout first.