Add missing G_GNUC_PRINTF attributes

This allows callers to be checked for mismatches between format string
and arguments, and also means gcc can assume that the format string and
the arguments match up correctly when forwarding them to functions
like g_strdup_vprintf, removing the need to suppress -Wformat-nonliteral
warnings.

Signed-off-by: Simon McVittie <smcv@collabora.com>
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