Add missing build dependency: libmail-message-perl

Add missing build dependency: libmail-message-perl

The post-build tests were failing on tens of errors like:

		t/07_bugreport.t ..................... Can't locate Mail/Message/Field.pm in @INC (you may need to install the Mail::Message::Field module) (@INC entries checked: lib /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.38.2 /usr/local/share/perl/5.38.2 /usr/lib/x86_64-linux-gnu/perl5/5.38 /usr/share/perl5 /usr/lib/x86_64-linux-gnu/perl-base /usr/lib/x86_64-linux-gnu/perl/5.38 /usr/share/perl/5.38 /usr/local/lib/site_perl .) at lib/Debbugs/MIME.pm line 57.
		BEGIN failed--compilation aborted at lib/Debbugs/MIME.pm line 57.
		Compilation failed in require at t/07_bugreport.t line 19.
		BEGIN failed--compilation aborted at t/07_bugreport.t line 19.

Add the missing dependency to overcome this and get the tests passing.

Note that the tests still emit multiple times this warning which
warrants additional follow-up later:

		t/07_bugreport.t ..................... 14/19
		Text::Xslate: get_info_from_db not implemented without schema at lib/Debbugs/Package.pm line 196.
			Debbugs::Package::_get_valid_version_info_from_db(Debbugs::Package={package=bar}) called at lib/Debbugs/Package.pm line 166
			Debbugs::Package::_build_valid_version_info(Debbugs::Package={package=bar}) called at /usr/share/perl5/MouseX/NativeTraits/MethodProvider/ArrayRef.pm line 315
			Debbugs::Package::_valid_version_info(Debbugs::Package={package=bar}) called at lib/Debbugs/Package.pm line 534
			Debbugs::Package::source_names(Debbugs::Package={package=bar}) called at lib/Debbugs/Package.pm line 525
			Debbugs::Package::_build_sources(Debbugs::Package={package=bar}) called at lib/Debbugs/Text.pm line 209
			eval {...} called at lib/Debbugs/Text.pm line 209
			Debbugs::Text::fill_in_template("template", "cgi/bugreport", "variables", HASH(0x559b9ebe1c10), "hole_var", HASH(0x559b9ebe2288)) called at ./cgi/bugreport.cgi line 403