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

Review URL: https://codereview.chromium.org/901273002

Cr-Commit-Position: refs/heads/master@{#314906}

third_party/libxml/libxml.gyp

@@ -237,6 +237,7 @@
                 'libraries': [
                   # We need dl for dlopen() and friends.
                   '-ldl',
+                  '-lm',
                 ],
               },
             }],