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.