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

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

Commit a2dabf0e1dda93c8 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.

Branch
------
REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/1598dc15bab35a9fd061babbae716777259d3147

Modified Files
--------------
src/bin/psql/print.c | 4 ++--
src/bin/psql/print.h | 2 +-
2 files changed, 3 insertions(+), 3 deletions(-)