Fixed __GCDWEBSERVER_LOGGING_HEADER__ check

When using __GCDWEBSERVER_LOGGING_HEADER__ the compiler ran into a missing
DCHECK definition. It looks like the #endif was misplaced.

Now checks for the NDEBUG and __GCDWEBSERVER_LOGGING_HEADER__ are
independent of each other and it's possible to #define custom LOG_xxx
macros in a separate header.