Add third_party/elfutils:libelf to the gn build.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index f9c011078990d2cc46dc40b319a842921d26cd94..8a40df38033dc2992934f0b4f9d7c8d9d694cbfb 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -69,6 +69,7 @@ group("root") {
     "//third_party/cacheinvalidation",
     "//third_party/cld",
     "//third_party/cld_2",
+    "//third_party/elfutils:libelf($host_toolchain)",

[brettw, 2014/10/10 17:48:04]

We only want to build this for desktop Linux, which means it probably shouldn't
be in this list. I suspect that this target is one that should only be brought
in "organically" when something else depends on it rather than always being
built. Is something like that coming in a followup?

[simonb (inactive), 2014/10/10 18:00:47]

Gone, then (though this makes it awkward to test with gn gen ...).

The thing that depends on this -- tools/relocation_packer -- doesn't yet build
with gn, but when I have that working then yes, libelf will be drawn in to the
host build via dependencies.

     "//third_party/ffmpeg",
     "//third_party/flac",
     "//third_party/harfbuzz-ng",