Add missing export/unexport directives

Test: treehugger
Bug: 172923994
Change-Id: I70494a02d37d149042a4d5772f1cf14b8bbcc06e
diff --git a/androidmk/parser/parser.go b/androidmk/parser/parser.go
index c14910a..5afef65 100644
--- a/androidmk/parser/parser.go
+++ b/androidmk/parser/parser.go
@@ -553,12 +553,14 @@
 	"else",
 	"endef",
 	"endif",
+	"export",
 	"ifdef",
 	"ifeq",
 	"ifndef",
 	"ifneq",
 	"include",
 	"-include",
+	"unexport",
 }
 
 var functions = [...]string{