Handle systems missing <sys/cdefs.h>

This is a non-portable header, and we cannot expect it to be provided by
the system libc (e.g. musl). We just need and rely on declaration that
we have defined ourselves in our own <bsd/sys/cdefs.h>. So we switch to
only ever assume that.

Fixes: https://bugs.freedesktop.org/105281

include/bsd/sys/queue.h

@@ -33,7 +33,11 @@
 #ifndef LIBBSD_SYS_QUEUE_H
 #define LIBBSD_SYS_QUEUE_H
 
+#ifdef LIBBSD_OVERLAY
 #include <sys/cdefs.h>
+#else
+#include <bsd/sys/cdefs.h>
+#endif
 
 /*
  * This file defines four types of data structures: singly-linked lists,

include/bsd/sys/tree.h

@@ -30,7 +30,11 @@
 #ifndef LIBBSD_SYS_TREE_H
 #define LIBBSD_SYS_TREE_H
 
+#ifdef LIBBSD_OVERLAY
 #include <sys/cdefs.h>
+#else
+#include <bsd/sys/cdefs.h>
+#endif
 
 /*
  * This file defines data structures for different types of trees: