ext4l: Add linux/utsname.h stub header

Add a stub header for linux/utsname.h needed by mmp.c and other
ext4 files.

Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

include/linux/utsname.h

@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_UTSNAME_H
+#define _LINUX_UTSNAME_H
+
+/* Stub for linux/utsname.h */
+struct new_utsname {
+	char sysname[65];
+	char nodename[65];
+	char release[65];
+	char version[65];
+	char machine[65];
+	char domainname[65];
+};
+
+struct uts_namespace {
+	struct new_utsname name;
+};
+
+extern struct uts_namespace init_uts_ns;
+
+#endif /* _LINUX_UTSNAME_H */