.gitignore: ignore external debug symbols from GCC on macOS

When Git is build with a "real" GCC on macOS [1], or at least with GCC
installed via Homebrew, and CFLAGS includes the '-g' option (and our
default CFLAGS does), then by default GCC writes the debug symbols
into external files under '<binary>.dSYM/' directories (e.g.
'git-daemon.dSYM/', 'git.dSYM/', etc.).

Update '.gitignore' to ignore these directories, so they don't clutter
the output of 'git status'.  Furthermore, these build artifacts then
won't trigger build failures on Travis CI via b92cb86ea1 (travis-ci:
check that all build artifacts are .gitignore-d, 2017-12-31) once one
of the following patches updates our CI build scripts to use a real
GCC in the 'osx-gcc' build job.

[1] On macOS the default '/usr/bin/gcc' executable is not a real GCC,
    but merely a compatibility wrapper around Clang:

      $ gcc --version
      Configured with: --prefix=<...>
      Apple LLVM version 9.0.0 (clang-900.0.39.2)
      <...>

    So even though 'make CC=gcc' does indeed execute a command called
    'gcc', in the end Git will be built with Clang all the same.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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