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>

tools/kwbimage.c

@@ -728,7 +728,7 @@ static void kwbimage_set_header(void *ptr, struct stat *sbuf, int ifd,
 	FILE *fcfg;
 	void *image = NULL;
 	int version;
-	size_t headersz;
+	size_t headersz = 0;
 	uint32_t checksum;
 	int ret;
 	int size;