gstreamer: Peephole optimisation for SpiceFormatForGStreamer

Reduce structure length using static allocated string inside the
structure.
This will also avoid using .data.rel.ro section and relocations
reducing even more library size.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Acked-by: Pavel Grunt <pgrunt@redhat.com>

server/gstreamer-encoder.c

@@ -40,7 +40,7 @@
 
 typedef struct {
     SpiceBitmapFmt spice_format;
-    const char *format;
+    char format[8];
     uint32_t bpp;
     uint32_t depth;
     uint32_t endianness;