tools/imximage.c: Fix compiling warning

Convert set_hdr_func(struct imx_header *imxhdr) to set_hdr_func(void)
to get rid of the warning

warning: ‘imxhdr’ is used uninitialized in this function

Signed-off-by: York Sun <yorksun@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>