scripts: kernel-doc: use :c:union when needed

Sphinx C domain code after 3.2.1 will start complaning if :c:struct
would be used for an union type:

	.../Documentation/gpu/drm-kms-helpers:352: ../drivers/video/hdmi.c:851: WARNING: C 'identifier' cross-reference uses wrong tag: reference name is 'union hdmi_infoframe' but found name is 'struct hdmi_infoframe'. Full reference name is 'union hdmi_infoframe'. Full found name is 'struct hdmi_infoframe'.

So, let's address this issue too in advance, in order to
avoid future issues.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/6e4ec3eec914df62389a299797a3880ae4490f35.1603791716.git.mchehab+huawei@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/kernel-doc

@@ -1092,7 +1092,11 @@ sub output_struct_rst(%) {
 	print "\n\n.. c:type:: " . $name . "\n\n";
     } else {
 	my $name = $args{'struct'};
-	print "\n\n.. c:struct:: " . $name . "\n\n";
+	if ($args{'type'} eq 'union') {
+	    print "\n\n.. c:union:: " . $name . "\n\n";
+	} else {
+	    print "\n\n.. c:struct:: " . $name . "\n\n";
+	}
     }
     print_lineno($declaration_start_line);
     $lineprefix = "   ";