nemo-seahorse: Fix build with gpgme >= 2.0.0

[andreasstieger]

nemo-extensions fails to build with gnupg >= 2.0.0. From https://build.opensuse.org/package/live_build_log/openSUSE:Factory/nemo-extensions/standard/x86_64

[   46s] ../tool/seahorse-pgp-operation.c:303:10: error: ‘GPGME_EVENT_NEXT_TRUSTITEM’ undeclared (first use in this function); did you mean ‘GPGME_EVENT_NEXT_KEY’?
[   46s]   303 |     case GPGME_EVENT_NEXT_TRUSTITEM:
[   46s]       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~

From the gpgme 2.0.0 NEWS:

• Removed the entire trustlist feature which worked anyway only for a
  short period in 2003. [T4834]

openSUSE bug boo#1246849
Identical Arch patch: https://gitlab.archlinux.org/archlinux/packaging/packages/nemo-extensions/-/commit/c90afb05f353f498341b20bf86bdb0004f4dd168