Declare the ORCA entry points in the unit test gpopt mock

gpopt_mock.c defines SerializeDXLPlan(), GPOPTOptimizedPlan(),
LibraryVersion(), EnableXform(), DisableXform(), InitGPOPT() and
TerminateGPOPT(), but the only declarations of them are in the ORCA C++
headers, which a C file cannot include.  With the warning flags configure
picks, every definition is then a -Wmissing-prototypes warning, and
src/Makefile.custom makes it an error:

  gpopt_mock.c:10:1: error: no previous prototype for 'SerializeDXLPlan'
  [-Werror=missing-prototypes]

Since gpopt_mock.o is part of every test program's MOCK_OBJS, "make
check" in any src/backend/*/test directory stops there.

"make unittest-check" from the top of the tree does not see this,
because it passes CFLAGS=-DUNITTEST on the command line, which replaces
the configured warning flags altogether; so CI stays green while running
a single directory's tests, as one does while writing them, fails.

Declare the seven functions in the mock, and give InitGPOPT() and
TerminateGPOPT() the (void) parameter list the declarations have.

src/test/unit/mock/gpopt_mock.c

diff --git a/src/test/unit/mock/gpopt_mock.c b/src/test/unit/mock/gpopt_mock.c
index 4f70eb0..fd270d5 100644
--- a/src/test/unit/mock/gpopt_mock.c
+++ b/src/test/unit/mock/gpopt_mock.c
@@ -6,6 +6,15 @@
 #include "nodes/plannodes.h"
 #include "optimizer/orcaopt.h"
 
+/* Declared in C++ headers (gpopt/utils/funcs.h, gpopt/CGPOptimizer.h) */
+extern char *SerializeDXLPlan(Query *pquery);
+extern PlannedStmt *GPOPTOptimizedPlan(Query *pquery, bool pfUnexpectedFailure, OptimizerOptions *opts);
+extern Datum LibraryVersion(void);
+extern Datum EnableXform(PG_FUNCTION_ARGS);
+extern Datum DisableXform(PG_FUNCTION_ARGS);
+extern void InitGPOPT(void);
+extern void TerminateGPOPT(void);
+
 char *
 SerializeDXLPlan(Query *pquery)
 {
@@ -42,13 +51,13 @@
 }
 
 void
-InitGPOPT ()
+InitGPOPT(void)
 {
 	elog(ERROR, "mock implementation of InitGPOPT called");
 }
 
 void
-TerminateGPOPT ()
+TerminateGPOPT(void)
 {
 	elog(ERROR, "mock implementation of TerminateGPOPT called");
 }