libqmi: fix autobuild failures

The libqmi build specifies -Werror in the CFLAGS by default. This causes
build failures such as the following.

  http://autobuild.buildroot.net/results/070/070449299aa9025c4bc6769524a6e22d66c92a65/



The -Werror flag can be disabled using a configuration switch.

[Peter: use =no instead and add a comment explaining why]
Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>