Add printf format attributes

When compiling with gcc, we already include the attribute on check_msg()
to give compiler warnings about mismatches between printf() like format
strings and the corresponding arguments.  This patch adds similar
attributes for lexical_error() and die().

Suggested-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>