vsock/uapi: fix linux/vm_sockets.h userspace compilation errors

If a userspace application just include <linux/vm_sockets.h> will fail
to build with the following errors:

    /usr/include/linux/vm_sockets.h:182:39: error: invalid application of ‘sizeof’ to incomplete type ‘struct sockaddr’
      182 |         unsigned char svm_zero[sizeof(struct sockaddr) -
          |                                       ^~~~~~
    /usr/include/linux/vm_sockets.h:183:39: error: ‘sa_family_t’ undeclared here (not in a function)
      183 |                                sizeof(sa_family_t) -
          |

Include <sys/socket.h> for userspace (guarded by ifndef __KERNEL__)
where `struct sockaddr` and `sa_family_t` are defined.
We already do something similar in <linux/mptcp.h> and <linux/if.h>.

Fixes: d021c34 ("VSOCK: Introduce VM Sockets")
Reported-by: Daan De Meyer <daan.j.demeyer@gmail.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20250623100053.40979-1-sgarzare@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/uapi/linux/vm_sockets.h

@@ -17,6 +17,10 @@
 #ifndef _UAPI_VM_SOCKETS_H
 #define _UAPI_VM_SOCKETS_H
 
+#ifndef __KERNEL__
+#include <sys/socket.h>        /* for struct sockaddr and sa_family_t */
+#endif
+
 #include <linux/socket.h>
 #include <linux/types.h>