gitignore: Add generated docbook stylesheets

I switched them over to generated files in commit
9fb81c6e54a2fe05c0ad0b877fd32f30358e3877, but forgot
to add them to the ignore file.

Gbp-Dch: ignore

.gitignore

@@ -24,10 +24,15 @@ gtest-*
 !/po/apt-all.pot
 
 # Vendor files
-
 **/vendor/sources.list
 **/LC_MESSAGES/
 
+# docbook stylesheets (generated from .cmake.in)
+docbook-html-style.xsl
+manpage-style.xsl
+docbook-text-style.xsl
+
+# Programs
 **/ftparchive/apt-ftparchive
 **/cmdline/apt
 **/cmdline/apt-cache