examples: Initialize @pos in domtop.c

This is a zero-cost workaround for a bug in GCC 8.3.0 which causes the
compilation to fail, because the compiler thinks that the value might be used
uninitialized even though it clearly cannot be.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>

examples/domtop/domtop.c

@@ -206,7 +206,7 @@ print_cpu_usage(size_t cpu,
     }
 
     for (i = 0; i < ncpus; i++) {
-        size_t pos;
+        size_t pos = 0;
         double usage;
 
         /* check if the vCPU is in the maps */