Try <linux/a.out.h> if <a.out.h> is not present At least musl ships the former but not the latter.
diff --git a/include/bsd/nlist.h b/include/bsd/nlist.h
index 2730237..8531f7a 100644
--- a/include/bsd/nlist.h
+++ b/include/bsd/nlist.h
@@ -28,7 +28,12 @@
#define LIBBSD_NLIST_H
#include <sys/cdefs.h>
+
+#if __has_include(<a.out.h>)
#include <a.out.h>
+#elif __has_include(<linux/a.out.h>)
+#include <linux/a.out.h>
+#endif
__BEGIN_DECLS
extern int nlist(const char *filename, struct nlist *list);