mjit.c: drop obsoleted duplicated declaration

of mjit_worker(). It was needed when mjit_worker.c is separated from
mjit.c, but it's now just included.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64306 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

mjit.c

@@ -355,7 +355,7 @@ init_header_filename(void)
 # else
         "-L"
 # endif
-	;
+        ;
     const size_t libpathflag_len = sizeof(libpathflag) - 1;
 #endif
 
@@ -538,8 +538,6 @@ system_tmpdir(void)
 static int
 start_worker(void)
 {
-    extern void mjit_worker(void);
-
     stop_worker_p = FALSE;
     worker_stopped = FALSE;