relibc: dirent/stdlib/unistd headers must include <features.h> for __deprecated

The relibc bump added #[deprecated] to readdir_r (and others); cbindgen emits
'__deprecated' (globdefs deprecated=__deprecated), which is #defined in
features.h. But dirent/stdlib/unistd generate with no_includes and never pulled
features.h, so a C unit that includes <dirent.h> before anything that defines
__deprecated fails: 'expected ; before int' (broke htop and any <dirent.h>
consumer). Added '#include <features.h>' to each header's cbindgen after_includes.

src/header/stdlib/cbindgen.toml

@@ -15,6 +15,7 @@
 sys_includes = ["stddef.h", "sys/wait.h", "stdint.h"]
 include_guard = "_RELIBC_STDLIB_H"
 after_includes = """
+#include <features.h>  // __deprecated
 #include <bits/open-flags.h> // for O_* constants, shared with fcntl.h
 """
 trailer = """