avoid warnings in test-select.c: addendum

[Jim Meyering]

Nearly forgot this.
Technically you might argue that it is not really required, since
  ./gnulib-tool --create-testdir --with-tests --test select
succeeds without it.  (some dependent module pulls in that commonly
required file)  But it's better to make it explicit, so I've folded
this in:

diff --git a/modules/select-tests b/modules/select-tests
index 6019b44..566e4d9 100644
--- a/modules/select-tests
+++ b/modules/select-tests
@@ -1,4 +1,5 @@
 Files:
+tests/macros.h
 tests/signature.h
 tests/test-select.c
 tests/test-select-fd.c