Remove "using namespace std" from dwarf2reader.h

Remove "using namespace std" from dwarf2reader.h. Using-directives are
forbidden by the style guide, and are bad practice in headers even under
style rules that tolerate this construct.

This fixes warnings such as:

In file included from dwarf2reader.cc:34:
dwarf2reader.h:53:17: warning: using namespace directive in global context in
header [-Wheader-hygiene]

Committed r862

[Mark Mentovai]

(TBR)

[jimb]

LGTM

I don't like 'using namespace' much either; I don't know what I was thinking
when I added that, or approved its addition.

I would kind of rather see 'using std::string' than meaningless "std::" prefixes
all over, but that's fine.