fix segmentation fault when plugins are loaded

fix segmentation fault when plugins are loaded

Function rpc_plugin_register_library() is modifying the struct's contents in
list_add() call. That said, the rpc_plugin struct should not be defined as
const. This caused segmentation fault when built on up-to-date Arch Linux.

Signed-off-by: Luka Perkov <luka@openwrt.org>