tools/virtio: fix up compiler.h stub

Add #undef __user before and after including compiler_types.h to avoid
redefinition warnings when compiling with system headers that also
define __user. This allows tools/virtio to build without warnings.

Additionally, stub out __must_check

Created using Cursor CLI.

Message-ID: <56424ce95c72cb4957070a7cd3c3c40ad5addaee.1764873799.git.mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tools/virtio/linux/compiler.h

@@ -2,7 +2,11 @@
 #ifndef LINUX_COMPILER_H
 #define LINUX_COMPILER_H
 
+/* Avoid redefinition warnings */
+#undef __user
 #include "../../../include/linux/compiler_types.h"
+#undef __user
+#define __user
 
 #define WRITE_ONCE(var, val) \
 	(*((volatile typeof(val) *)(&(var))) = (val))
@@ -35,4 +39,6 @@
 	__v;								\
 })
 
+#define __must_check
+
 #endif