plugin: fair_tree: fix installation path

Problem: the sched priority_mod_fair_tree.so plugin is installed into
$libdir directly instead of under $libdir/flux/sched as required to be
found by the flux-module frontend command.

The build system already exports $schedplugindir, so move the
priority_mod_fair_tree.so from lib_LTLIBRARIES to schedplugin_LTLIBRARIES.

Fixes #344

sched/priority/modified_fair_tree/Makefile.am

@@ -11,7 +11,7 @@ libconvenience_la_CXXFLAGS = \
 libconvenience_la_LIBADD = $(top_builddir)/resrc/libflux-resrc.la \
 	$(FLUX_CORE_LIBS) $(JANSSON_LIBS) $(CZMQ_LIBS)
 
-lib_LTLIBRARIES = priority_mod_fair_tree.la
+schedplugin_LTLIBRARIES = priority_mod_fair_tree.la
 # Dummy C++ source to cause C++ linking.
 nodist_EXTRA_priority_mod_fair_tree_la_SOURCES = dummy.cpp
 priority_mod_fair_tree_la_SOURCES =