Don't specify LOCAL_STRIP_MODULE for static libraries

After I92a838b07fe4116d5a4b8521fe1ce8d44e6e84e2 in build/,
LOCAL_STRIP_MODULE := keep_symbols will be honored for prebuilts, which
will trigger an error if it is set on a static library.  Only set
LOCAL_STRIP_MODULE for binaries and shared libraries.

Change-Id: I4e17a78afa7c5810dbf9003166c14a1ff3a59c9e
diff --git a/cc/androidmk.go b/cc/androidmk.go
index 499736d..95250a6 100644
--- a/cc/androidmk.go
+++ b/cc/androidmk.go
@@ -79,7 +79,9 @@
 func (library *libraryLinker) AndroidMk(ret *android.AndroidMkData) {
 	library.baseLinker.AndroidMk(ret)
 
-	library.stripper.AndroidMk(ret)
+	if !library.static() {
+		library.stripper.AndroidMk(ret)
+	}
 
 	ret.Extra = append(ret.Extra, func(w io.Writer, outputFile android.Path) error {
 		var exportedIncludes []string