isaspec: encode.py: Include assert.h

pack_field(..) makes use of assert(..).

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27491>

src/compiler/isaspec/encode.py

@@ -340,6 +340,7 @@ template = """\
  * IN THE SOFTWARE.
  */
 
+#include <assert.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <util/bitset.h>