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>

tools/zunitc/src/zuc_junit_reporter.c

@@ -53,6 +53,12 @@
 
 #define ISO_8601_FORMAT "%Y-%m-%dT%H:%M:%SZ"
 
+#if LIBXML_VERSION >= 20904
+#define STRPRINTF_CAST
+#else
+#define STRPRINTF_CAST BAD_CAST
+#endif
+
 /**
  * Internal data.
  */
@@ -68,7 +74,7 @@ static void
 set_attribute(xmlNodePtr node, const char *name, int value)
 {
 	xmlChar scratch[MAX_64BIT_STRLEN + 1] = {};
-	xmlStrPrintf(scratch, sizeof(scratch), BAD_CAST "%d", value);
+	xmlStrPrintf(scratch, sizeof(scratch), STRPRINTF_CAST "%d", value);
 	xmlSetProp(node, BAD_CAST name, scratch);
 }