Replace char copyright[] by static const char copyright[].

It seems the latter is used throughout the tree.

usr.bin/find/main.c

@@ -31,7 +31,7 @@
  */
 
 #ifndef lint
-char copyright[] =
+static const char copyright[] =
 "@(#) Copyright (c) 1990, 1993, 1994\n\
 	The Regents of the University of California.  All rights reserved.\n";
 #endif /* not lint */

usr.bin/lex/main.c

@@ -27,7 +27,7 @@
  */
 
 #ifndef lint
-char copyright[] =
+static const char copyright[] =
 "@(#) Copyright (c) 1990 The Regents of the University of California.\n\
  All rights reserved.\n";
 #endif /* not lint */

usr.sbin/mount_portalfs/mount_portalfs.c

@@ -31,7 +31,7 @@
  */
 
 #ifndef lint
-char copyright[] =
+static const char copyright[] =
 "@(#) Copyright (c) 1992, 1993, 1994\n\
 	The Regents of the University of California.  All rights reserved.\n";
 #endif /* not lint */