cffi: fix build failure wrt libyang 6.3.0

Since libyang commit b6ccfc9e0165 ("ly_array UPDATE public API sized
array"), libyang.h does not include plugins.h anymore. So the build
fails with -Werror, because lyplg_add_extension_plugin() is not
declared.

Include plugins.h directly. It is present in all supported versions,
so nothing changes with older libyang.

Closes: #175
Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Signed-off-by: Samuel Gauthier <samuel.gauthier@6wind.com>

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