rust: Add header library support to rust_bindgen.

Allow rust_bindgen modules to define dependencies that only provide
headers and may not necessarily need to be linked in.

Bug: 161141999
Test: Soong tests pass.
Test: Example module has appropriate include flags when compiling.
Change-Id: Ic9ce8b1204008ad8dcb18766c914e48bb292d485

rust/protobuf.go

@@ -53,7 +53,7 @@ type ProtobufProperties struct {
 	Proto_flags []string `android:"arch_variant"`
 
 	// List of libraries which export include paths required for this module
-	Header_libs []string `android:"arch_variant"`
+	Header_libs []string `android:"arch_variant,variant_prepend"`
 }
 
 type protobufDecorator struct {