genksyms: add printf format attribute to error_with_pos()

When compiling with -Wsuggest-attribute=format in HOSTCFLAGS, gcc
complains that error_with_pos() may be declared with a printf format
attribute:

    scripts/genksyms/genksyms.c:726:3: warning: function might be
    possible candidate for ‘gnu_printf’ format attribute
    [-Wsuggest-attribute=format]
       vfprintf(stderr, fmt, args);
       ^~~~~~~~

This would allow catching printf-format errors at compile time in
callers to error_with_pos(). Add this attribute.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>