Bugfix: removed double-stripping of parentheses

Bugfix: removed double-stripping of parentheses

GVariant keeps body inside structs. This fact is kept hidden from
libdbus user. Thus, struct parentheses have to be added, removed
in proper places. There was one such place in
_dbus_type_reader_gvariant_init(). However, it was not sufficient
and get_const_signature() also got stripping code. Unfortunately,
I didn't notice that stripping should be removed in the first
of these functions. This commit fixes that.

Change-Id: I5c2e6c7c8212d283fd6da1cc998a2711038905f3