string: mark makenstr as unused

This function will be relevant only if we ever optimize some
size-limited string function in assembler, which would warrant the user
of memarray to validate size/alignment combinations.

src/string/misc.c

@@ -13,6 +13,7 @@ static size_t makestr(char *buf, size_t size)
 }
 
 /* Make a string of size slightly smaller than size bytes (including NUL) */
+__attribute__((unused))
 static size_t makenstr(char *buf, size_t size)
 {
     size_t n = max(3 * size / 4, 4);