Always create both variants for kernel headers

kernel_headers exports header files from TARGET_DEVICE_KERNEL_HEADERS,
TARGET_BOARD_KERNEL_HEADERS, and TARGET_PRODUCT_KERNEL_HEADERS. All of
them are bound to vendor, so they will always have both variants.

Bug: 157106227
Test: manual
Change-Id: I903b7ddbee21f63174dbba47bc6c093b90d1b42d
diff --git a/cc/cc.go b/cc/cc.go
index 8eabff5..85e1bde 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -3106,10 +3106,19 @@
 		}
 	} else if vendorSpecific && String(m.Properties.Sdk_version) == "" {
 		// This will be available in /vendor (or /odm) only
-		// We assume that modules under proprietary paths are compatible for
-		// BOARD_VNDK_VERSION. The other modules are regarded as AOSP, or
-		// PLATFORM_VNDK_VERSION.
-		if isVendorProprietaryPath(mctx.ModuleDir()) {
+
+		// kernel_headers is a special module type whose exported headers
+		// are coming from DeviceKernelHeaders() which is always vendor
+		// dependent. They'll always have both vendor variants.
+		// For other modules, we assume that modules under proprietary
+		// paths are compatible for BOARD_VNDK_VERSION. The other modules
+		// are regarded as AOSP, which is PLATFORM_VNDK_VERSION.
+		if _, ok := m.linker.(*kernelHeadersDecorator); ok {
+			vendorVariants = append(vendorVariants,
+				platformVndkVersion,
+				boardVndkVersion,
+			)
+		} else if isVendorProprietaryPath(mctx.ModuleDir()) {
 			vendorVariants = append(vendorVariants, boardVndkVersion)
 		} else {
 			vendorVariants = append(vendorVariants, platformVndkVersion)