travis: Attempt to fix src/lxc/cmd/lxc_init.c:251: undefined reference to `pthread_sigmask

.travis.yml

@@ -10,6 +10,7 @@ script:
  - ./coccinelle/run-coccinelle.sh -i
  - git diff --exit-code
  - export CFLAGS="-Wall -Werror"
+ - export LDFLAGS="-pthread -lpthread"
  - ./autogen.sh
  - rm -Rf build
  - mkdir build