build: adapt to libyang 6.3.0+ header reshuffle

Since upstream commit
CESNET/libyang@b6ccfc9, `libyang.h` no
longer pulls in `plugins.h`, and therefore `lyplg_add_extension_plugin`
was not declared properly.

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>

Author: jktjkt

cffi/source.c

@@ -4,6 +4,7 @@
  */
 
 #include <libyang/libyang.h>
+#include <libyang/plugins.h>
 #include <libyang/version.h>
 
 #if LY_VERSION_MAJOR * 10000 + LY_VERSION_MINOR * 100 + LY_VERSION_MICRO < 50000