Add extern "C" to function declarations in VPI tests.

These are necessary to link the executables. So far we have been saved
by one of the generated headers forward declaring these functions with
extern "C", but changing that header would break these tests.

test_regress/t/t_vpi_get.cpp

@@ -202,7 +202,7 @@ int mon_check_props() {
     return 0;
 }
 
-int mon_check() {
+extern "C" int mon_check() {
     // Callback from initial block in monitor
     if (int status = mon_check_props()) return status;
     return 0;  // Ok

test_regress/t/t_vpi_memory.cpp

@@ -213,7 +213,7 @@ void _mon_check_memory() {
     }
 }
 
-int mon_check() {
+extern "C" int mon_check() {
     // Callback from initial block in monitor
     _mon_check_memory();
     return errors;

test_regress/t/t_vpi_param.cpp

@@ -203,7 +203,7 @@ int _mon_check_param() {
     return status;
 }
 
-int mon_check() {
+extern "C" int mon_check() {
     // Callback from initial block in monitor
     if (int status = _mon_check_param()) return status;
     return 0;  // Ok

test_regress/t/t_vpi_unimpl.cpp

@@ -176,7 +176,7 @@ int _mon_check_unimpl(p_cb_data cb_data) {
     return 0;  // Ok
 }
 
-int mon_check() {
+extern "C" int mon_check() {
     // Callback from initial block in monitor
     if (int status = _mon_check_unimpl(NULL)) return status;
     return 0;  // Ok

test_regress/t/t_vpi_var.cpp

@@ -638,7 +638,7 @@ int _mon_check_vlog_info() {
     return 0;
 }
 
-int mon_check() {
+extern "C" int mon_check() {
     // Callback from initial block in monitor
 #ifdef TEST_VERBOSE
     printf("-mon_check()\n");