perlvars.h: remove extraneous semicolon

None of the other PERLVARI declarations adds an extra semicolon,
so don't put one on the new shutdownhook either.  Removing it
prevents the following compiler message on VMS:

PERLVARI(G, shutdownhook, shutdown_proc_t, &Perl_noshutdownhook);
................................................................^
%CC-I-EXTRASEMI, Extraneous semicolon.
at line number 387 in file DKA0:[CRAIG.blead]perlvars.h;1

The message doesn't cause compile failures in and of itself, but
it pollutes the output of probes in some extensions and causes
capability detection to go off the rails in, for example,
Time::HiRes.

perlvars.h

@@ -384,7 +384,7 @@ PERLVAR(G, user_prop_mutex, perl_mutex)    /* Mutex for manipulating
  * This is needed for tearing down threads::shared and probably
  * not be used for anything else.
  */
-PERLVARI(G, shutdownhook, shutdown_proc_t, &Perl_noshutdownhook);
+PERLVARI(G, shutdownhook, shutdown_proc_t, &Perl_noshutdownhook)
 
 /* these record the best way to perform certain IO operations while
  * atomically setting FD_CLOEXEC. On the first call, a probe is done