rust: Change BinderBindings.h extension to .hpp

The build system now makes assumptions about how to interpret the
wrapper source for rust_bindgen modules. As BinderBindings.h is a C++
header, change it's extension to .hpp so the build system automatically
interprets it as a C++ header.

Bug: 163580541
Test: m libbinder_ndk_bindgen
Change-Id: I6ffd26b64ec388ec5a89b9138caeef37015ad973