fix order of defines with default values.
fix order of defines with default values.

This fixes a problem when CONFIG_XML_HEAP_BUFFERS was not defined.
The CONFIG_XML_THREAD_SAFE section is using this define which causes a
warnings otherwise.

gergap/libroxml,gergap/libroxml,gergap/libroxml,gergap/libroxml,gergap/libroxml