variable: Add header_libs to product_variables.arc

Add header_libs to product_variables.arc struct, which is needed for
cc_library_headers.

Bug: 151048523
Test: Image builds.
Change-Id: I1e1af35fb2c3a55454c9b70acfa10abb5d65ef61
diff --git a/android/variable.go b/android/variable.go
index dd000ad..e17c558 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -124,6 +124,7 @@
 		Arc struct {
 			Cflags            []string `android:"arch_variant"`
 			Exclude_srcs      []string `android:"arch_variant"`
+			Header_libs       []string `android:"arch_variant"`
 			Include_dirs      []string `android:"arch_variant"`
 			Shared_libs       []string `android:"arch_variant"`
 			Static_libs       []string `android:"arch_variant"`