lib: smbios: remove redundant next_header()

next_header() and get_next_header() only differ in how the const attribute
is used. One function taking a const parameter and returning a non-const is
good enough.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>