ASoC: soc-topology: cleanup cppcheck warning at soc_tplg_process_headers()

This patch cleanups below cppcheck warning.

sound/soc/soc-topology.c:2576:27: style: The scope of the variable 'hdr' can be reduced. [variableScope]
 struct snd_soc_tplg_hdr *hdr;
                          ^

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87sfzswflw.wl-kuninori.morimoto.gx@renesas.com


Signed-off-by: Mark Brown <broonie@kernel.org>