Update to kernel headers v5.2.6.

Also, fix a bug in update_all.py when the syscalls file does not change.

Test: Builds and boots on a taimen.
Change-Id: If85b00daef2c176f804a0861894f5bbca9c6d5df
diff --git a/libc/kernel/uapi/linux/bpf.h b/libc/kernel/uapi/linux/bpf.h
index 11df994..3f09421 100644
--- a/libc/kernel/uapi/linux/bpf.h
+++ b/libc/kernel/uapi/linux/bpf.h
@@ -548,6 +548,7 @@
   char name[BPF_OBJ_NAME_LEN];
   __u32 ifindex;
   __u32 gpl_compatible : 1;
+  __u32 : 31;
   __u64 netns_dev;
   __u64 netns_ino;
   __u32 nr_jited_ksyms;