LLSD-14: Optional entry points need conditional decls turned on.

Changeset 07cd70e75473 moved LLSD::outstandingCount() and allocationCount() to
free functions so we could turn their visibility on/off via LLSD_DEBUG_INFO.
But on some platforms, without proper LL_COMMON_API declarations visible when
we compile llsd.cpp, those free functions lack proper linkage directives.
Declare LLSD_DEBUG_INFO in llsd.cpp so that when the llcommon library is
built, the free functions get proper linkage -- independent of compilations of
LLSD consumers.