fix: tools: kwbimage.c: Initialize headersz to suppress warning

When building with my toolchain (4.8.2):
CROSS_COMPILE=/home/lukma/work/ptxdist/toolchains/arm/OSELAS.Toolchain-2013.12.0/arm-v7a-linux-gnueabi/gcc-4.8.2-glibc-2.18-binutils-2.24-kernel-3.12-sanitized/bin/arm-v7a-linux-gnueabi-

I see following WARNING:
tools/kwbimage.c: In function "kwbimage_set_header":
tools/kwbimage.c:803:8: warning: "headersz" may be used uninitialized in this function [-Wmaybe-uninitialized]
  memcpy(ptr, image, headersz);
        ^
This fix aims to suppress it.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>