kconfig/symbol.c: use correct pointer type argument for sizeof

sym_arr is of type struct symbol **.
So in malloc we need sizeof(struct symbol *).

The problem was indicated by coccinelle.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>