Add default argument to symbol_get.
Add default argument to symbol_get.

Under gcc, "#define foo(a) #a\n foo()" gives "", whereas it gives nothing
with vc++.

	* src/object/symbol.hh(symbol_get): Add default value of "" to
	  argument.

aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi,urbiforge/urbi,aldebaran/urbi,aldebaran/urbi,urbiforge/urbi