It isn't in the repository. It gets built automatically when you build something that requires it.
You can build it manually; go to /usr/pkgsrc/pkgtools/osabi and make clean && make install
It's possible you have a wrong version of it installed.
pkg_info | grep osabi will tell you. If so, pkg_delete it first. In that case, you probably won't have to build it manually, building x11-links will build it.
You could install x11-links as a binary package with pkgin. If it isn't in the repository, chances are you won't be able to build it as is, either.
If you put the repository in mk.conf, you can make bin-install in the x11/rofi directory and it will install whatever binary packages it can; greatly speeding up the process.
I have BINPKG_SITES= ${PKGSRCDIR}/packages/${MACHINE_ARCH}/${OS_VERSION} in my /etc/mk.conf. I know in some cases you have to tweak that.
Edit: you don't want that. You want that variable to be the same as whatever you have in /usr/pkg/etc/pkgin/repositories.conf