[gdb/cli] Skip string copy in source_cache::ensure

In function source_cache::ensure we have:
...
 	      std::ostringstream output;
	      ...
	      contents = output.str ();
...
The last line causes an unnecessary string copy.

C++20 allows us to skip it, like this:
...
	      contents = std::move (output).str ();
...

Use the more efficient solution.

Tested on x86_64-linux.

Reviewed-By: Lancelot Six <lancelot.six@amd.com>