t9902: 'send-email' test case requires PERL

The oneline notwithstanding, 13374987 (completion: use _gitcompbuiltin
for format-patch, 2018-11-03) changed also the way send-email options
are completed, by asking the git send-email command itself what options
it offers.

Necessarily, this must fail when built with NO_PERL because send-email
itself is a Perl script. Which means that we need the PERL prerequisite
for the send-email test case in t9902.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9902-completion.sh

@@ -1539,7 +1539,7 @@ test_expect_success 'complete tree filename with metacharacters' '
 	EOF
 '
-test_expect_success 'send-email' '
+test_expect_success PERL 'send-email' '
 	test_completion "git send-email --cov" "--cover-letter " &&
 	test_completion "git send-email ma" "master "
 '