Move struct plugin_data_struct to plugin.c

It isn't needed anywhere except plugin.c.  The typedef can disappear.
Also make a forward declaraion for ld_plugin_input_file in plugin.h
so that this header can be used without first including plugin-api.h.

bfd/
	* plugin.h (struct ld_plugin_input_file): Forward declare.
	(struct plugin_data_struct): Move to..
	* plugin.c: ..here.
	(add_symbols): Size plugin_data without using type.
	* archive.c: Don't include plugin-api.h.
	* elflink.c: Likewise.
	* format.c: Likewise.
binutils/
	* ar.c: Don't include plugin-api.h or ansidecl.h.  Only
	include plugin.h when BFD_SUPPORTS_PLUGINS.
	* nm.c: Don't include plugin-api.h.  Only include plugin.h
	when BFD_SUPPORTS_PLUGINS.
	* objcopy.c: Likewise.
ld/
	* ldfile.c: Don't include plugin-api.h.
	* ldmain.c: Likewise.