TagType: reduce the enum size to 1 byte

Reduce the TagItem overhead.

src/tag/TagItem.hxx

@@ -35,7 +35,7 @@ struct TagItem {
 	/**
 	 * the value of this tag; this is a variable length string
 	 */
-	char value[sizeof(long)];
+	char value[sizeof(long) - sizeof(type)];
 
 	TagItem() = default;
 	TagItem(const TagItem &other) = delete;

src/tag/TagType.h

@@ -20,10 +20,21 @@
 #ifndef MPD_TAG_TYPE_H
 #define MPD_TAG_TYPE_H
 
+#ifdef __cplusplus
+#include <stdint.h>
+#endif
+
 /**
  * Codes for the type of a tag item.
  */
-enum tag_type {
+enum tag_type
+#ifdef __cplusplus
+/* the size of this enum is 1 byte; this is only relevant for C++
+   code; the only C sources including this header don't use instances
+   of this enum, they only refer to the integer values */
+: uint8_t
+#endif
+	{
 	TAG_ARTIST,
 	TAG_ARTIST_SORT,
 	TAG_ALBUM,