misc: vmw_vmci_defs: Mark 'struct vmci_handle VMCI_ANON_SRC_HANDLE' as __maybe_unused

vmw_vmci_defs.h is included by multiple source files. Some of which
do not make use of 'struct vmci_handle VMCI_ANON_SRC_HANDLE' rendering
it unused. Ensure the compiler knows that this is in fact intentional
by marking it as __maybe_unused. This fixes the following W=1 warnings:

In file included from drivers/misc/vmw_vmci/vmci_context.c:8:
include/linux/vmw_vmci_defs.h:162:33: warning: ‘VMCI_ANON_SRC_HANDLE’ defined but not used [-Wunused-const-variable=]
162 | static const struct vmci_handle VMCI_ANON_SRC_HANDLE = {
| ^~~~~~~~~~~~~~~~~~~~
In file included from drivers/misc/vmw_vmci/vmci_datagram.c:8:
include/linux/vmw_vmci_defs.h:162:33: warning: ‘VMCI_ANON_SRC_HANDLE’ defined but not used [-Wunused-const-variable=]
162 | static const struct vmci_handle VMCI_ANON_SRC_HANDLE = {
| ^~~~~~~~~~~~~~~~~~~~

Cc: George Zhang <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>
Acked-by: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

include/linux/vmw_vmci_defs.h

···
  */
 #define VMCI_ANON_SRC_CONTEXT_ID   VMCI_INVALID_ID
 #define VMCI_ANON_SRC_RESOURCE_ID  VMCI_INVALID_ID
-static const struct vmci_handle VMCI_ANON_SRC_HANDLE = {
+static const struct vmci_handle __maybe_unused VMCI_ANON_SRC_HANDLE = {
 	.context = VMCI_ANON_SRC_CONTEXT_ID,
 	.resource = VMCI_ANON_SRC_RESOURCE_ID
 };