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.

json-c.sym

@@ -41,6 +41,8 @@ JSONC_PRIVATE {
     printbuf_new;
     printbuf_reset;
     sprintbuf;
+    # Used by tests:
+    _json_c_strerror;
 };
 
 JSONC_0.14 {