util: Do not use PRIx64 macro

It breaks the build on Mingw32,  because PRIx64 is coming
from the Win32 headers, but virAsprintf uses the gnulib printf.

src/util/virrandom.c

@@ -123,8 +123,8 @@ virRandomGenerateWWN(char **wwn,
         return -1;
     }
 
-    if (virAsprintf(wwn, "5" "%s%09" PRIx64,
-                    oui, virRandomBits(36)) < 0) {
+    if (virAsprintf(wwn, "5" "%s%09llx", oui,
+                    (unsigned long long)virRandomBits(36)) < 0) {
         virReportOOMError();
         return -1;
     }