Add core_handler and pid2md into git ignore

`core_handler` and `pid2md` are always untracked after `make`. They are
the output of build. We should ignore them.

```
Untracked files:
  (use "git add <file>..." to include in what will be committed)
    src/tools/linux/core_handler/core_handler
    src/tools/linux/pid2md/pid2md
```

Change-Id: Ib20fdaf6b902f4de0b336ab22423c7271f8dc26c
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/2678993
Reviewed-by: Mike Frysinger <vapier@chromium.org>

.gitignore

@@ -44,8 +44,10 @@ lib*.a
 /src/processor/minidump_dump
 /src/processor/minidump_stackwalk
 /src/tools/linux/core2md/core2md
+/src/tools/linux/core_handler/core_handler
 /src/tools/linux/dump_syms/dump_syms
 /src/tools/linux/md2core/minidump-2-core
+/src/tools/linux/pid2md/pid2md
 /src/tools/linux/symupload/minidump_upload
 /src/tools/linux/symupload/sym_upload
 /src/tools/mac/dump_syms/dump_syms