Define PyInit_cmodule so the Windows build will pass

Author: mdboom

pyperformance/data-files/benchmarks/bm_ctypes/cmodule.c

@@ -26,3 +26,13 @@ void void_foo_constchar(const char* str) {
 
 }
 
+PyMODINIT_FUNC
+PyInit_cmodule(void) {
+  // DELIBERATELY EMPTY
+
+  // This isn't actually a Python extension module (it's used via ctypes), so
+  // this entry point function will never be called. However, we are utilizing
+  // setuptools to build it, and on Windows, setuptools explicitly passes the
+  // flag /EXPORT:PyInit_cmodule, so it must be defined.
+  return NULL;
+}