doc: doxygen: Add openthread module header to INPUT

Add openthread module header to INPUT due to dependencies to symbols in
`zephyr/net/openthread.h`

Signed-off-by: Adrian Gielniewski <adrian.gielniewski@nordicsemi.no>

doc/zephyr.doxyfile.in

@@ -981,6 +981,7 @@ INPUT                  = @ZEPHYR_BASE@/doc/_doxygen/mainpage.md \
                          @ZEPHYR_BASE@/subsys/testsuite/ztest/include/ \
                          @ZEPHYR_BASE@/subsys/secure_storage/include/ \
                          @ZEPHYR_BASE@/subsys/fs/zms/zms_priv.h \
+                         @ZEPHYR_BASE@/modules/openthread/include/openthread.h \
 
 # 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