docs: filesystems: add fs/open.c to api-summary

Include fs/open.c in filesystems/api-summary.rst to provide its
exported APIs.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260104204530.518206-1-rdunlap@infradead.org>

Author: rddunlap

Documentation/filesystems/api-summary.rst

@@ -56,6 +56,9 @@ Other Functions
 .. kernel-doc:: fs/namei.c
    :export:
 
+.. kernel-doc:: fs/open.c
+   :export:
+
 .. kernel-doc:: block/bio.c
    :export: