Fix vecstim.mod extern declarations for NEURON 9 compatibility

The explicit parameter types added in 7f43d9b conflict with
declarations already provided by NEURON >= 8.2.0 headers, causing
an "ambiguating new declaration" error in the generated C++.
Guard them with #ifndef NRN_VERSION_GTEQ_8_2_0, consistent with
the nrn_random_pick fix in 8699516.

Author: apdavison

pyNN/neuron/nmodl/vecstim.mod

@@ -113,10 +113,12 @@ NET_RECEIVE (w) {
 
 
 VERBATIM
+#ifndef NRN_VERSION_GTEQ_8_2_0
 extern double* vector_vec(void *);
 extern int vector_capacity(void *);
 extern void* vector_arg(int);
-ENDVERBATIM     
+#endif
+ENDVERBATIM
 
 PROCEDURE element() {
 VERBATIM