[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.