Clean symbols directory during installclean

Installclean is used in some build server builds when switching between
different TARGET_BUILD_APPS, remove the symbols directory so it doesn't
accumulate old symbols files.

Bug: 186507256
Test: m && m installclean && m
Change-Id: I316703cd0985ee555959a6dcdb39d5a702ab0332
diff --git a/ui/build/cleanbuild.go b/ui/build/cleanbuild.go
index 6ba497c..19b5690 100644
--- a/ui/build/cleanbuild.go
+++ b/ui/build/cleanbuild.go
@@ -144,7 +144,8 @@
 		productOut("odm"),
 		productOut("odm_dlkm"),
 		productOut("sysloader"),
-		productOut("testcases"))
+		productOut("testcases"),
+		productOut("symbols"))
 }
 
 // Since products and build variants (unfortunately) shared the same