media: vicodec: get_next_header is static

drivers/media/platform/vicodec/vicodec-core.c:drivers/media/platform/vicodec/vicodec-core.c:210:23:  warning: symbol 'get_next_header' was not declared. Should it be static?
drivers/media/platform/vicodec/vicodec-core.c:210:23: warning: no previous prototype for 'get_next_header' [-Wmissing-prototypes]
 enum vb2_buffer_state get_next_header(struct vicodec_ctx *ctx, u8 **pp, u32 sz)
                       ^~~~~~~~~~~~~~~

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>