[gdb/build] Fix unused var in corelow.c

[gdb/build] Fix unused var in corelow.c

On x86_64-linux, with gcc 7.5.0 and CFLAGS/CXXFLAGS="-O0 -g -Wall" I ran into
a build breaker:
...
gdb/corelow.c: In member function ‘void mapped_file_info::add(const char*, const char*, const char*, std::vector<mem_range>&&, const bfd_build_id*)’:
gdb/corelow.c:1822:27: error: unused variable ‘it’ [-Werror=unused-variable]
   const auto [it, inserted]
                           ^
...

Fix this by dropping the variable it.

Tested on x86_64-linux.

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