[gdb/build] Fix unused var in lookup_dwo_unit_in_dwp

On x86_64-linux, with gcc 7.5.0 I ran into a build breaker:
...
gdb/dwarf2/read.c: In function ‘dwo_unit* lookup_dwo_unit_in_dwp()’:
gdb/dwarf2/read.c:7403:22: error: unused variable ‘inserted’ \
  [-Werror=unused-variable]
    auto [it, inserted] = dwo_unit_set.emplace (std::move (dwo_unit));
                      ^
...

Fix this by dropping the unused variable.

Tested on x86_64-linux, by completing a build.

gdb/dwarf2/read.c

@@ -7400,7 +7400,7 @@ lookup_dwo_unit_in_dwp (dwarf2_per_bfd *per_bfd,
 	  std::lock_guard<std::mutex> guard (dwp_file->loaded_cutus_lock);
 #endif
 
-	  auto [it, inserted] = dwo_unit_set.emplace (std::move (dwo_unit));
+	  auto it = dwo_unit_set.emplace (std::move (dwo_unit)).first;
 	  return it->get ();
 	}