security/rnp: fix build using c++19 include files

The build failed with:

libcxx: error: implicit instantiation of undefined template
'std::char_traits<unsigned char>

This was caused by a restriction of char_traits to a subset of the
integer types that used to be supported (e.g. char, wchar_t), which
no longer includes the type unsigned char.

There has been no new release, yet, but upstream fixed this issue
in commit 20419f7.

PR: 282348
Reported by: jkim
Obtained from: https://github.com/rnpgp/rnp/commit/20419f7