add sources line for static_assert_test_fail (make dist wouldn't work otherwise)

credits for finding this issue go to the Freiburg nightly builds

[[Imported from SVN: r5745]]

dune/common/test/Makefile.am

@@ -51,6 +51,7 @@ nullptr_test_fail_SOURCES = nullptr-test.cc
 nullptr_test_fail_CPPFLAGS = -DFAIL
 
 static_assert_test_SOURCES = static_assert_test.cc
+static_assert_test_fail_SOURCES = static_assert_test_fail.cc
 
 fassigntest_SOURCES = fassigntest.cc