Export stub library compiler flags to soong_injection.

These are used by the "stub_library" cc toolchain feature.

Test: CI
Bug: 207812332
Change-Id: I392e51f32c92b30b30e85658e7627e16190bdbad
diff --git a/cc/config/bp2build.go b/cc/config/bp2build.go
index 4797acc..982b436 100644
--- a/cc/config/bp2build.go
+++ b/cc/config/bp2build.go
@@ -22,6 +22,7 @@
 	"strings"
 
 	"android/soong/android"
+
 	"github.com/google/blueprint"
 )
 
@@ -162,6 +163,10 @@
 	exportedStringListVars.Set(name, value)
 }
 
+func ExportStringList(name string, value []string) {
+	exportedStringListVars.Set(name, value)
+}
+
 type exportedStringListDictVariables map[string]map[string][]string
 
 func (m exportedStringListDictVariables) Set(k string, v map[string][]string) {