maint: fix 'make syntax-check' failure

* doc/.gitignore: Add a newline so the sc_prohibit_empty_lines_at_EOF
syntax-check no longer fails.

diff --git a/doc/.gitignore b/doc/.gitignore
line changes: +1/-1
index 6e95f6d..b52d18e
--- a/doc/.gitignore
+++ b/doc/.gitignore

@@ -1,4 +1,4 @@
 /version.texi
 /gendocs_template
 /gendocs_template_min
-/manual/ 
\ No newline at end of file
+/manual/