versioning macro is exported from prebuilt stubs as well

This change fixes a bug that the versioning macro (__LIBNAME_API__) is
omitted for prebuilts providing a stub.

Bug: 175166063
Test: m nothing

Change-Id: I1cce5ab58ef245622861200ec0d8b0f84e3178ed
diff --git a/cc/library.go b/cc/library.go
index 06b9905..71b1f45 100644
--- a/cc/library.go
+++ b/cc/library.go
@@ -1274,9 +1274,7 @@
 	}
 
 	// Add stub-related flags if this library is a stub library.
-	if library.buildStubs() && !library.skipAPIDefine {
-		library.reexportFlags("-D" + versioningMacroName(ctx.Module().(*Module).ImplementationModuleName(ctx)) + "=" + library.stubsVersion())
-	}
+	library.exportVersioningMacroIfNeeded(ctx)
 
 	// Propagate a Provider containing information about exported flags, deps, and include paths.
 	library.flagExporter.setProvider(ctx)
@@ -1284,6 +1282,14 @@
 	return out
 }
 
+func (library *libraryDecorator) exportVersioningMacroIfNeeded(ctx android.BaseModuleContext) {
+	if library.buildStubs() && !library.skipAPIDefine {
+		name := versioningMacroName(ctx.Module().(*Module).ImplementationModuleName(ctx))
+		ver := library.stubsVersion()
+		library.reexportFlags("-D" + name + "=" + ver)
+	}
+}
+
 // buildStatic returns true if this library should be built as a static library.
 func (library *libraryDecorator) buildStatic() bool {
 	return library.MutatedProperties.BuildStatic &&