.travis.yml: disable linux-user build for gcov

.travis.yml: disable linux-user build for gcov

Currently the default testing doesn't exercise the linux-user builds
so there is no point spending time building them. We may want to
enable a separate gcov build once linux-user testing is re-enabled
although it's likely to report very low coverage.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>