vd_agent: Do not define structures without fields

According to C standard the size of a structure without fields is
not defined (see
https://stackoverflow.com/questions/21851930/size-of-empty-structure-in-c-and-c).
As we almost only use GCC currently these structures are of size 0.
This can became a problem if 2 sides talking a given protocol use
these structures.
To avoid such possible mismatch (for instance having one side C and
the other C++ which defines these structures as having 1 as size) add
an empty (0-size array) field.
Although a 0-size array is not considered also that portable we use
extensively this feature with different compilers never encountering
issues.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Uri Lublin <uril@redhat.com>

spice/vd_agent.h

@@ -234,6 +234,7 @@ typedef struct SPICE_ATTR_PACKED VDAgentClipboardRelease {
     uint8_t selection;
     uint8_t __reserved[sizeof(uint32_t) - 1 * sizeof(uint8_t)];
 #endif
+    uint8_t dummy_empty_field[0]; /* C/C++ compatibility */
 } VDAgentClipboardRelease;
 
 typedef struct SPICE_ATTR_PACKED VDAgentMaxClipboard {