metricsd: Only define the targets on linux.

libchromeos (dependency of metricsd) is only defined for linux.
All metrics target must only be defined on mac too or the build system
will complain.

BUG: 22879597
Change-Id: I5bee816e001518a8053d1dfc7b39cca81a4314fd

metricsd/Android.mk

@@ -14,6 +14,8 @@
 
 LOCAL_PATH := $(call my-dir)
 
+ifeq ($(HOST_OS),linux)
+
 metrics_cpp_extension := .cc
 libmetrics_sources := \
   c_metrics_library.cc \
@@ -110,3 +112,5 @@ LOCAL_SHARED_LIBRARIES := $(metrics_shared_libraries) \
 LOCAL_SRC_FILES := $(metrics_daemon_sources)
 LOCAL_STATIC_LIBRARIES := metrics_daemon_protos
 include $(BUILD_EXECUTABLE)
+
+endif # HOST_OS == linux