Add missing const qualifier to compile with clang/llvm.

BUG: 21298442
Change-Id: Iad211237fdb0128d8f4490419687bf1da1cd261c
diff --git a/libs/hwui/utils/RingBuffer.h b/libs/hwui/utils/RingBuffer.h
index 6d0a06b..d822cb2 100644
--- a/libs/hwui/utils/RingBuffer.h
+++ b/libs/hwui/utils/RingBuffer.h
@@ -31,7 +31,7 @@
     RingBuffer() {}
     ~RingBuffer() {}
 
-    constexpr size_t capacity() { return SIZE; }
+    constexpr size_t capacity() const { return SIZE; }
     size_t size() { return mCount; }
 
     T& next() {