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
diff --git a/rust/bindgen.go b/rust/bindgen.go
index 7cc0fc8..35a807b 100644
--- a/rust/bindgen.go
+++ b/rust/bindgen.go
@@ -258,5 +258,6 @@
 
 	deps.SharedLibs = append(deps.SharedLibs, b.ClangProperties.Shared_libs...)
 	deps.StaticLibs = append(deps.StaticLibs, b.ClangProperties.Static_libs...)
+	deps.HeaderLibs = append(deps.StaticLibs, b.ClangProperties.Header_libs...)
 	return deps
 }