nostrdb: strblock: add typedef

I don't technically need this but it helps a lot on the swift side
of things since I already have code that uses this identifier of a
similar structure

Signed-off-by: William Casarin <jb55@jb55.com>

nostrdb/src/str_block.h

@@ -4,9 +4,9 @@
 
 #include <inttypes.h>
 
-struct ndb_str_block {
+typedef struct ndb_str_block {
 	const char *str;
 	uint32_t len;
-};
+} str_block_t;
 
 #endif