doc: zephyr: add missing Doxygen input folder

Align with list of inputs in upstream Zephyr.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

Diff for: doc/zephyr/zephyr.doxyfile.in

@@ -978,7 +978,8 @@ INPUT                  = @DOCSET_SOURCE_BASE@/doc/_doxygen/mainpage.md \
                          @DOCSET_SOURCE_BASE@/include/ \
                          @DOCSET_SOURCE_BASE@/lib/libc/minimal/include/ \
                          @DOCSET_SOURCE_BASE@/subsys/testsuite/include/ \
-                         @DOCSET_SOURCE_BASE@/subsys/testsuite/ztest/include/
+                         @DOCSET_SOURCE_BASE@/subsys/testsuite/ztest/include/ \
+                         @DOCSET_SOURCE_BASE@/subsys/secure_storage/include/
 
 # This tag can be used to specify the character encoding of the source files
 # that Doxygen parses. Internally Doxygen uses the UTF-8 encoding. Doxygen uses