TagTable: un-inline the two functions

Reduce header dependencies.

src/decoder/FlacMetadata.cxx

@@ -28,7 +28,7 @@
 #include <glib.h>
 
 #include <assert.h>
-#include <stdlib.h>
+#include <string.h>
 
 static bool
 flac_find_float_comment(const FLAC__StreamMetadata *block,

src/decoder/XiphTags.cxx

@@ -24,5 +24,5 @@ const struct tag_table xiph_tags[] = {
 	{ "tracknumber", TAG_TRACK },
 	{ "discnumber", TAG_DISC },
 	{ "album artist", TAG_ALBUM_ARTIST },
-	{ NULL, TAG_NUM_OF_ITEM_TYPES }
+	{ nullptr, TAG_NUM_OF_ITEM_TYPES }
 };