security/gpgme-cpp: Fix constness

The 1.15.0 release of gpgme-cpp adds a Signature::operator< which is missing
a const. In 6a6d2a27648, Signature got an operator< . This is used in
*security/libkleo*, for instance, to sort the signatures for display.

The build failure looks like this (trimmed for brevity, on 13-):

/usr/include/c++/v1/algorithm:715:71: error: invalid operands to binary
expression ('const GpgME::UserID::Signature' and 'const
GpgME::UserID::Signature')

bool operator()(const _T1& __x, const _T1& __y) const {return __x < __y;}

/wrkdirs/usr/ports/security/libkleo/work/libkleo-20.12.0/src/models/
useridlistmodel.cpp:203:14: note: in instantiation of function template
specialization 'std::__1::sort<GpgME::UserID::Signature>' requested here

std::sort(sigs.begin(), sigs.end());

PR: 252283
Submitted by: adridg