libxml2: Add missing dependency on libm

libxml2: Add missing dependency on libm

The library uses functions from "math.h". This fixes the embedded build
with -Wl,-z,defs.

BUG=445627
TEST=linux_ecs_ozone tryjob
TBR=cpu

Committed: https://crrev.com/e1393cf52aa9e7b8cbcf0a4776030253e8de2583
Cr-Commit-Position: refs/heads/master@{#314906}

[spang, 2015-02-05 21:21:53]

spang@chromium.org changed reviewers:
+ dnicoara@chromium.org

[dnicoara, 2015-02-05 21:22:55]

lgtm

[spang, 2015-02-05 22:03:59]

The CQ bit was checked by spang@chromium.org

[commit-bot: I haz the power, 2015-02-05 22:05:11]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/901273002/1

[commit-bot: I haz the power, 2015-02-05 23:10:02]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-05 23:11:00]

Patchset 1 (id:??) landed as
https://crrev.com/e1393cf52aa9e7b8cbcf0a4776030253e8de2583
Cr-Commit-Position: refs/heads/master@{#314906}