Re-enable compilation with MS Visual Studio, as "ngspice/config.h"

is not available.
Add-on to commit
4881b2973 ("Fix Bug #772 - "ngspice + icarus verilog cosim: searches for ivlng.so                 in /usr/local/lib/ngspice ...". Use the configured library directory, NGSPICELIBDIR.", 2025-04-11)

src/xspice/verilog/icarus_shim.c

@@ -18,7 +18,12 @@
 
 #include "coroutine_shim.h"
 
+#ifdef _MSC_VER
+#include "../../../visualc/src/include/ngspice/config.h"
+#else
 #include "ngspice/config.h"  // For NGSPICELIBDIR
+#endif
+
 #include "ngspice/cmtypes.h" // For Digital_t
 #include "ngspice/cosim.h"