nanoapp_postprocess: Exclude from non-Linux builds

Fixes the Mac OSX SDK build by restricting the build of
nanoapp_postprocess to Linux host platforms only, due to missing libelf
dependency on other platforms.

Change-Id: I461df05fdbf3f3f72579a5aee733203050618305
diff --git a/util/nanoapp_postprocess/Android.mk b/util/nanoapp_postprocess/Android.mk
index f9cb94f..89a62a3 100644
--- a/util/nanoapp_postprocess/Android.mk
+++ b/util/nanoapp_postprocess/Android.mk
@@ -14,6 +14,10 @@
 # limitations under the License.
 #
 
+# libelf is not available in the Mac build as of June 2016, but we currently
+# only need to use this tool on Linux, so exclude this from non-Linux builds
+ifeq ($(HOST_OS),linux)
+
 LOCAL_PATH := $(call my-dir)
 
 include $(CLEAR_VARS)
@@ -39,3 +43,5 @@
 LOCAL_MODULE_TAGS := optional
 
 include $(BUILD_HOST_EXECUTABLE)
+
+endif # linux