Don't memset non-POD types: struct btrace_insn

Don't memset non-POD types: struct btrace_insn

struct btrace_insn is not a POD [1] so we shouldn't be using memset to
initialize it [2].

Use list-initialization instead, wrapped in a "pt insn to btrace insn"
function, which looks like just begging to be added next to the
existing pt_reclassify_insn/pt_btrace_insn_flags functions.

[1] - because its field "flags" is not POD, because enum_flags has a
non-trivial default ctor.

gdb/ChangeLog:
2017-04-25  Pedro Alves  <palves@redhat.com>

	* btrace.c (pt_btrace_insn_flags): Change parameter type to
	reference.
	(pt_btrace_insn): New function.
	(ftrace_add_pt): Remove memset call and use pt_btrace_insn.