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.

CMakeLists.txt

@@ -398,7 +398,6 @@ set(JSON_C_HEADERS
     ${PROJECT_SOURCE_DIR}/json_object_private.h
     ${PROJECT_SOURCE_DIR}/random_seed.h
     ${PROJECT_SOURCE_DIR}/strerror_override.h
-    ${PROJECT_SOURCE_DIR}/strerror_override_private.h
     ${PROJECT_SOURCE_DIR}/math_compat.h
     ${PROJECT_SOURCE_DIR}/snprintf_compat.h
     ${PROJECT_SOURCE_DIR}/strdup_compat.h