Get rid of scribbling on a const variable in psql's print.c.

Commit a2dabf0e had the bright idea that it could modify a "const"
global variable if it merely casted away const from a pointer.  This does
not work on platforms where the compiler puts "const" variables into
read-only storage.  Depressingly, we evidently have no such platforms in
our buildfarm ... an oversight I have now remedied.  (The one platform
that is known to catch this is recent OS X with -fno-common.)

Per report from Chris Ruprecht.  Back-patch to 9.5 where the bogus
code was introduced.

src/bin/psql/print.c

@@ -98,7 +98,7 @@ const printTextFormat pg_asciiformat_old =
 };
 
 /* Default unicode linestyle format */
-const printTextFormat pg_utf8format;
+printTextFormat pg_utf8format;
 
 typedef struct unicodeStyleRowFormat
 {
@@ -3412,7 +3412,7 @@ get_line_style(const printTableOpt *opt)
 void
 refresh_utf8format(const printTableOpt *opt)
 {
-	printTextFormat *popt = (printTextFormat *) &pg_utf8format;
+	printTextFormat *popt = &pg_utf8format;
 
 	const unicodeStyleBorderFormat *border;
 	const unicodeStyleRowFormat *header;

src/bin/psql/print.h

@@ -163,7 +163,7 @@ typedef struct printQueryOpt
 
 extern const printTextFormat pg_asciiformat;
 extern const printTextFormat pg_asciiformat_old;
-extern const printTextFormat pg_utf8format;
+extern printTextFormat pg_utf8format;	/* ideally would be const, but... */
 
 
 extern void disable_sigpipe_trap(void);