zunitc/junit-reporter: Silence pointer-sign warning

zunitc/junit-reporter: Silence pointer-sign warning

/usr/x86_64-pc-linux-gnu/include/libxml2/libxml/xmlstring.h:35:18:
warning: pointer targets in passing argument 3 of 'xmlStrPrintf' differ
in signedness [-Wpointer-sign]
 #define BAD_CAST (xmlChar *)
                  ^
tools/zunitc/src/zuc_junit_reporter.c:77:41: note: in expansion of macro
'BAD_CAST'
  xmlStrPrintf(scratch, sizeof(scratch), BAD_CAST %d, value);
                                         ^~~~~~~~
/usr/x86_64-pc-linux-gnu/include/libxml2/libxml/xmlstring.h:98:17: note:
expected 'const char *' but argument is of type 'xmlChar * {aka unsigned
char *}'
                 xmlStrPrintf             (xmlChar *buf,

Warning produced by GCC 5.3, 6.1 and Clang 3.8.

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>
Tested-by: Armin Krezović <krezovic.armin@gmail.com>