docs: kernel_abi.py: add sphinx build dependencies

Ensure that Sphinx-build will handle the files parsed by
get_abi.pl as dependencies. This way, if they are touched,
the ABI output will be regenerated.

Reported-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/10bc3d3bc74f514a539cd3b48b9d287d2b6f99e2.1648290305.git.mchehab@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Author: mchehab

Documentation/sphinx/kernel_abi.py

@@ -128,6 +128,7 @@ def runCmd(self, cmd, **kwargs):
         return out
 
     def nestedParse(self, lines, fname):
+        env = self.state.document.settings.env
         content = ViewList()
         node = nodes.section()
 
@@ -154,6 +155,9 @@ def nestedParse(self, lines, fname):
                     self.do_parse(content, node)
                     content = ViewList()
 
+                    # Add the file to Sphinx build dependencies
+                    env.note_dependency(os.path.abspath(f))
+
                 f = new_f
 
                 # sphinx counts lines from 0