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 <georgezhang@vmware.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200708125711.3443569-2-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/vmw_vmci_defs.h

@@ -159,7 +159,7 @@ static inline bool vmci_handle_is_invalid(struct vmci_handle 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
 };