Fix typo and Mac build

FAILED:
out/target/product/sailfish/obj/ETC/treble_sepolicy_tests_intermediates/treble_sepolicy_tests
Error: library-path out/host/darwin-x86/lib64/libsepolwrap.so
does not exist

Note, fixing here instead of reverting to avoid reverting
changes in CTS.

Test: ctate testing on Mac
Change-Id: I95f483b152d9bece1a16267cbc49eedb1f902990
diff --git a/Android.mk b/Android.mk
index 173b983..11a693d 100644
--- a/Android.mk
+++ b/Android.mk
@@ -177,7 +177,7 @@
 endif
 
 # Library extension for host-side tests
-ifeq ($(HOSTOS),darwin)
+ifeq ($(HOST_OS),darwin)
 SHAREDLIB_EXT=dylib
 else
 SHAREDLIB_EXT=so