tools: automake: fix MAINTAINERCLEANFILES warnings

tools: automake: fix MAINTAINERCLEANFILES warnings

It seems that, for whatever reason in this case,
the function "vardef ($var, $cond)" does not work
while "$var->def ($cond)" does work for conditionals.

Also, do not define it conditionally when defined unconditionally.
Even though the reordering patch would make that functionally sound,
it still throws a warning which can cause a build to fail
when warnings are treated as errors.

Instead, just add BUILT_SOURCES to every existing case
rather than only when BUILT_SOURCES is defined.

Fixes: 6d2bfe50d3 ("tools/automake: control all cleaning with clean variables")
Signed-off-by: Michael Pratt <mcpratt@pm.me>
Link: https://github.com/openwrt/openwrt/pull/19593
Signed-off-by: Robert Marko <robimarko@gmail.com>