Stop linking a copy of _json_c_strerror() (strerror_override.c) into the tests.

That hasn't been needed since since commit 6068d3f, which changed that code to
 check an env var instead ("_JSON_C_STRERROR_ENABLE").
Fixes issue #812, about dup symbols in static builds with clang.

tests/test_strerror.c

@@ -2,7 +2,6 @@
 #undef NDEBUG
 #endif
 #include "strerror_override.h"
-#include "strerror_override_private.h"
 
 #include <stdio.h>