bfd: fix incorrect type used in sizeof

Noticed in passing that we used 'sizeof (char **)' when calculating
the size of a list of 'char *' pointers.  Of course, this isn't really
going to make a difference anywhere, but we may as well be correct.

There should be no user visible changes after this commit.

bfd/ChangeLog:

	* archures.c (bfd_arch_list): Use 'char *' instead of 'char **'
	when calculating space for a string list.