diff --git a/tools/build/fcntl.h b/tools/build/fcntl.h
index 6aaf3eef2804..f59f5f7fb20a 100644
--- a/tools/build/fcntl.h
+++ b/tools/build/fcntl.h
@@ -29,6 +29,13 @@
 
 #include_next <fcntl.h>
 
+/*
+ * On FreeBSD fcntl.h indirectly brings in cdefs.h. On Linux with musl, it does
+ * not. Unconditionally inlude it here since there's no harm in including it
+ * multiple times since we use __BEGIN_DECLS and __END_DECLS from it below.
+ */
+#include <sys/cdefs.h>
+
 struct spacectl_range {
 	off_t	r_offset;
 	off_t	r_len;