See gcc info pages: "Macros with a Variable Number of Arguments."
The ISO-C stadanrd doesn't allow varargs macros to get zero or more
arguments. So this fixes the problem using the gcc extension.

A standard-compliant alternative would be to have "fmt" included in
the variadic part, like this:

  #define assert(cond, ...) \
        if (CONFIG_HAS_ASSERT && !(cond)) \
                __assert(__func__, __LINE__, 1 /* forever */, __VA_ARGS__)

But the reader wouldn't  now it's a fmt+args.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>