verify-sig.eclass: Append a trailing newline to gpg --verify output

Explicitly append a trailing newline to the output stream created
by "gpg --verify", to ensure it always contains one.  The FreePG
patchset to GPG corrects handling the trailing newlines in cleartext
signed messages, which means that GPG-created "incorrect" messages
no longer include a trailing newline character [1].  We are parsing
the checksum file via bash's "read" command that ignores the characters
after the final newline character, so we need to ensure that one is
always present after the ultimate checksum.

Adding an extra newline to vanilla GnuPG output causes no harm,
as the verification function will simply ignore the resulting empty
line.

[1] a4e8d23439/DIFF.md (handle-cleartext-signature-framework-csf-messages-correctly)

Signed-off-by: Michał Górny <mgorny@gentoo.org>

eclass/verify-sig.eclass

@@ -380,6 +380,7 @@ _gpg_verify_signed_checksums() {
 
 	verify-sig_verify_unsigned_checksums - "${algo}" "${files}" < <(
 		verify-sig_verify_message "${checksum_file}" - "${key}"
+		echo
 	)
 }