travis: set GCOV for zdtm.py too

It's required for collection full report for tests
which are executed in separate mount namespaces.
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

scripts/travis-tests

@@ -6,7 +6,8 @@ test "$1" = "x86_64" || exit 0
 apt-get update -qq
 apt-get install -qq protobuf-c-compiler libprotobuf-c0-dev libaio-dev libprotobuf-dev protobuf-compiler python-ipaddr libcap-dev libnl-3-dev
 chmod a+x $HOME
-make GCOV=1
+export GCOV=1
+make
 make -C test/zdtm
 python test/zdtm.py run -a -x 'cgroup*'