re-enable V source with parameter EXTERNAL

Fix bug #463

src/spicelib/devices/vsrc/vsrc.c

@@ -24,7 +24,7 @@ IFparm VSRCpTable[] = { /* parameters */
  IOP ("trnoise", VSRC_TRNOISE,   IF_REALVEC,"Transient noise description"),
  IOP ("trrandom", VSRC_TRRANDOM, IF_REALVEC,"random source description"),
 #ifdef SHARED_MODULE
- IOP ("external", VSRC_EXTERNAL, IF_REALVEC,"external source description"),
+ IOP ("external", VSRC_EXTERNAL, IF_STRING,"external source description"),
 #endif
  OPU ("pos_node",VSRC_POS_NODE,  IF_INTEGER,"Positive node of source"),
  OPU ("neg_node",VSRC_NEG_NODE,  IF_INTEGER,"Negative node of source"),