[2225_statistics] correct the module name according to renaming to counters.py

src/lib/python/isc/statistics/__init__.py

@@ -1 +1 @@
-from isc.statistics.counter import *
+from isc.statistics.counters import *

src/lib/python/isc/statistics/tests/Makefile.am

@@ -1,5 +1,5 @@
 PYCOVERAGE_RUN=@PYCOVERAGE_RUN@
-PYTESTS = counter_test.py
+PYTESTS = counters_test.py
 EXTRA_DIST = $(PYTESTS)
 EXTRA_DIST += testdata/test_spec1.spec
 EXTRA_DIST += testdata/test_spec2.spec