LOV_MAX_STRIPE_COUNT isn't available in Lustre < 2.4

LOV_MAX_STRIPE_COUNT, which darshan-lustre.c pulls in from lustre_user.h, wasn't defined in userspace until Lustre 2.4 or so. See:

• https://github.com/Xyratex/lustre-stable/commit/8a06a00535b929a8b5761764457be03a3308f6ec
• https://jira.hpdd.intel.com/browse/LU-1606

Just need to stuff in an #ifndef to define LOV_MAX_STRIPE_COUNT if it isn't already defined.