[obv] Fix broken build on Fedora 23.

Compiler complains about possible utilization of "symbol" which is member
of lang_def.
Initialization was added.

2016-07-07  Walfred Tedeschi  <walfred.tedeschi@intel.com>

gdb/ChangeLog:

	* cp-namespace.c (cp_lookup_bare_symbol): Initialize
	lang_this.symbol.