tests: fix compilation of shunloadtest

tests: fix compilation of shunloadtest

While local builds succeed fine, a build worker building in a
chroot environment is encountering the following error with
libvirt 3.0.0 release candidates

[  162s] shunloadtest.o: In function `main':
[  162s] /home/abuild/rpmbuild/BUILD/libvirt-3.0.0/tests/shunloadtest.c:110: undefined reference to `dlopen'
[  162s] /home/abuild/rpmbuild/BUILD/libvirt-3.0.0/tests/shunloadtest.c:114: undefined reference to `dlsym'
[  162s] /home/abuild/rpmbuild/BUILD/libvirt-3.0.0/tests/shunloadtest.c:133: undefined reference to `dlclose'
[  162s] /home/abuild/rpmbuild/BUILD/libvirt-3.0.0/tests/shunloadtest.c:111: undefined reference to `dlerror'
[  162s] /home/abuild/rpmbuild/BUILD/libvirt-3.0.0/tests/shunloadtest.c:115: undefined reference to `dlerror'
[  162s] /home/abuild/rpmbuild/BUILD/libvirt-3.0.0/tests/shunloadtest.c:116: undefined reference to `dlclose'

Fix by appending DLOPEN_LIBS to shunloadtest_LDADD.