util: add missing equal sign in initialization

Fix the build with clang:
util/virperf.c:86:27: error: use of GNU 'missing =' extension
    in designator [-Werror,-Wgnu-designator]
    [VIR_PERF_EVENT_MBML] {
                          ^
                          =

src/util/virperf.c

@@ -83,7 +83,7 @@ static struct virPerfEventAttr attrs[] = {
         .attrType = 0,
         .attrConfig = 2
     },
-    [VIR_PERF_EVENT_MBML] {
+    [VIR_PERF_EVENT_MBML] = {
         .attrType = 0,
         .attrConfig = 3
     },