ci.yml: ensure -Werror really gets used in all cases

-Werror wasn't actually being used when building the libraries, as the
libraries use CFLAGS_STLIB instead of CFLAGS.

Use CFLAGS_WARN, which gets included in both.

Note: -Werror can't just be passed to 'configure' like the other flags
are, as it interferes with some of the configure checks.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
1 file changed