Add LOCAL_EXPORT_CFLAGS for Soong

This lets Soong pass -isystem or -I as necessary (or potentially, even
other cflags in the future).

This is not available for Android.mk use, nor exposed directly to
Android.bp users.

Test: m -j
Change-Id: Id37d4692d5fbddce467bd777903b20169f44dd6e
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index c2ae8b8..37d8364 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -69,6 +69,7 @@
 LOCAL_EMMA_COVERAGE_FILTER:=
 LOCAL_EMMA_INSTRUMENT:=
 LOCAL_ERROR_PRONE_FLAGS:=
+LOCAL_EXPORT_CFLAGS:=
 LOCAL_EXPORT_C_INCLUDE_DEPS:=
 LOCAL_EXPORT_C_INCLUDE_DIRS:=
 LOCAL_EXPORT_HEADER_LIBRARY_HEADERS:=