libc: add linux headers

Yeah

Signed-off-by: kaguya <vpshinomiya@protonmail.com>

ext2_root/usr/include/bits/linux/linux_unistd.h

@@ -0,0 +1,21 @@
+#ifndef _BITS_LINUX_UNISTD_H
+#define _BITS_LINUX_UNISTD_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef __MLIBC_ABI_ONLY
+
+int dup3(int __fd, int __newfd, int __flags);
+int vhangup(void);
+int getdtablesize(void);
+int syncfs(int __fd);
+
+#endif /* !__MLIBC_ABI_ONLY */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _BITS_LINUX_UNISTD_H */