Fix 'unknown type name GC_INNER' compilation error (FreeBSD)

Fix 'unknown type name GC_INNER' compilation error (FreeBSD)
(fix commit f13a9559)

See issue #132.  The reason of compilation failure is GC_INNER macro
used before its definition.  The solution is to move
GC_FreeBSDGetDataStart prototype from gcconfig.h to gc_priv.h.

* include/private/gc_priv.h [DATASTART_USES_BSDGETDATASTART]
(GC_FreeBSDGetDataStart): Declare.
* include/private/gc_priv.h [DATASTART_USES_BSDGETDATASTART]
(DATASTART_IS_FUNC): Define macro.
* include/private/gcconfig.h [DATASTART_USES_BSDGETDATASTART]
(GC_FreeBSDGetDataStart, DATASTART_IS_FUNC): Remove.