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

cc/compiler.go

@@ -687,6 +687,9 @@ type RustBindgenClangProperties struct {
 	// list of shared libraries that provide headers for this binding.
 	Shared_libs []string `android:"arch_variant"`
 
+	// List of libraries which export include paths required for this module
+	Header_libs []string `android:"arch_variant,variant_prepend"`
+
 	// list of clang flags required to correctly interpret the headers.
 	Cflags []string `android:"arch_variant"`