PR12265, Compiling ld/ fails on Solaris 8

The fail was due to -Werror and headers included by dlfcn.h and
elf-bfd.h disagreeing about AT_DCACHEBSIZE and other AT_*.  Not a
serious problem obviously, since release versions of binutils don't
enable -Werror and the defines are not used.  Anyway, reduce the
number of files that might hit this problem by only including dlfcn.h
where it is needed.

	PR 12265
	* sysdep.h: Don't include dlfcn.h here.
	* plugin.c: Include it here.