[libxml2] Run Travis ASan tests with "sudo: required"

[Nick Wellnhofer <nwellnhof src gnome org>]

commit e61c82175e7b9289f751755dacc92d2c29fb6ffe
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Mon Apr 16 20:46:13 2018 +0200

    Run Travis ASan tests with "sudo: required"
    
    See https://github.com/travis-ci/travis-ci/issues/9033

 .travis.yml |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/.travis.yml b/.travis.yml
index da48143..469f9c5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -13,6 +13,7 @@ matrix:
       env: CFLAGS="-O2 -std=c89 -D_XOPEN_SOURCE=700 -Werror -Wno-error=unused-function"
     # clang with AddressSanitizer and UndefinedBehaviorSanitizer.
     - compiler: clang
+      sudo: required
       dist: trusty
       env: CONFIG="--without-python"
            CFLAGS="-O2 -g -fno-omit-frame-pointer -fsanitize=address,undefined 
-fno-sanitize=float-divide-by-zero,pointer-overflow -fno-sanitize-recover=all -Werror 
-Wno-error=unused-function -Wno-error=cast-align"