Allow libnv to be built externally using GCC.

GCC does not define _VA_LIST_DECLARED. It defines _VA_LIST_ and others.
This was causing the prototype to not be defined and leading to an error
later due to using nvlist_add_stringv(3) without a prototype in
nvlist_add_stringf(3).

This uses the same check as other va_list prototypes in the original
change in r279438.