add C wrapper for libmetrics

TEST=unit tests + build&test crosmetrics plugin for flimflam

Review URL: http://codereview.chromium.org/2832008
diff --git a/metrics/Makefile b/metrics/Makefile
index c8ba545..fa68a40 100644
--- a/metrics/Makefile
+++ b/metrics/Makefile
@@ -32,6 +32,7 @@
 	metrics_daemon.o \
 	metrics_daemon_test.o
 LIB_OBJS = \
+	c_metrics_library.o \
 	metrics_library.o
 TESTLIB_OBJS = \
 	metrics_library.o \