[gdb/testsuite] Fix return type of psymtab-parameter

As pointed out by Pedro Alves, psymtab-parameter testcase rely on the
return type being long. This patch revert the changes made in
f106e10e and change psymtab-parameter.cc
to return 0 long instead.

2017-11-29  Thomas Preud'homme  <thomas.preudhomme@arm.com>

gdb/testsuite/
	* gdb.cp/psymtab-parameter.cc (func): Change return type back to long.
	Return 0 as a long.
	* gdb.cp/psymtab-parameter.exp: Change func's return type back to long.