build: avoid too keen -Wsuggest-attribute=format warning

With gcc 5.3.1 20151207:

log.c: In function 'cs_format':
log.c:182:2: warning: function might be possible candidate for

           'gnu_printf' format attribute [-Wsuggest-attribute=format]
len = vsnprintf(str, QB_LOG_MAX_LEN, cs->format, ap_copy);
^

We certainly don't want to disable that warning globally so make use of
diagnostic pragmas for GCC instead in one instance that we cannot
annotate properly.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>