devel/cpprestsdk: allow consumers to find_package(cpprestsdk)

With sufficiently-new CMake settings, the CMake .config file for
cpprestsdk can't find Boost::system (because it no longer exists)
and then it fails. But cpprestsdk doesn't need it -- so patch
it out.

Upstream PR: https://github.com/microsoft/cpprestsdk/pull/1838
PR: 291874