gdb: remove unnecessary call to std::string constructor

I spotted this explicit call to std::string, which creates an
unnecessary temporary extra std::string, while calling emplace_back.
I'm not sure if it has any impact in an optimized build, maybe the
compiler elides it.  But still, it's unnecessary.

Change-Id: I873337ea91db29ac06267aff8fc12dcf52824cac
Approved-By: Tom Tromey <tom@tromey.com>