Suppress clang 7.0 warnings.

capacity_ is used in some build target, and not in others.
clang compiler gives warning on unused case, and also on
used case if declared as unused.

Test: make checkbuild
Change-Id: I1800d02129a7587096b6de19dd791d2af44596d4
diff --git a/runtime/indirect_reference_table.h b/runtime/indirect_reference_table.h
index 00184e2..d2093f2 100644
--- a/runtime/indirect_reference_table.h
+++ b/runtime/indirect_reference_table.h
@@ -187,6 +187,8 @@
  public:
   IrtIterator(IrtEntry* table, size_t i, size_t capacity) REQUIRES_SHARED(Locks::mutator_lock_)
       : table_(table), i_(i), capacity_(capacity) {
+    // capacity_ is used in some target; has warning with unused attribute.
+    UNUSED(capacity_);
   }
 
   IrtIterator& operator++() REQUIRES_SHARED(Locks::mutator_lock_) {