Fix compat.h for even older kernel headers

src/liburing/include/liburing/compat.h

@@ -4,24 +4,40 @@
 
 #include <linux/version.h>
 
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(5,1,0)
 #include <linux/time_types.h>
+#else
+struct __kernel_timespec {
+    int64_t tv_sec;
+    int64_t tv_nsec;
+};
+#endif
+
 /* <linux/time_types.h> is included above and not needed again */
 #define UAPI_LINUX_IO_URING_H_SKIP_LINUX_TIME_TYPES_H 1
 
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(5,6,0)
 #include <linux/openat2.h>
+#else
+struct open_how {
+    uint64_t flags;
+    uint16_t mode;
+    uint16_t __padding[3];
+    uint64_t resolve;
+};
+#endif
+
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4,14,0)
+typedef int __kernel_rwf_t;
+#endif
 
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(6,12,0)
-
 #include <linux/blkdev.h>
-
 #else
-
 #include <linux/ioctl.h>
-
 #ifndef BLOCK_URING_CMD_DISCARD
-#define BLOCK_URING_CMD_DISCARD                        _IO(0x12, 0)
-#endif
-
+#define BLOCK_URING_CMD_DISCARD _IO(0x12, 0)
+#endif
 #endif
 
 #endif