shared: fix coding-style for ring-buffer implementation

We use "typedef struct Ring Ring" with camel-case for internal objects.
So rename "struct ring" to "Ring".

src/test/test-ring.c

@@ -31,7 +31,7 @@
 
 static void test_ring(void) {
         static const char buf[8192];
-        struct ring r;
+        Ring r;
         size_t l;
         struct iovec vec[2];
         int s;