kconfig: sym_expand_string_value: allow for string termination when reallocing

When expanding a parameterised string we may run out of space, this
triggers a realloc.  When computing the new allocation size we do not
allow for the terminating '\0'.  Allow for this when calculating the new
length.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>