Fix missing parameter lists.

In C (before C23), a missing parameter list means “parameters are unknown”, not
“no parameters” (unlike C++).  See
https://en.cppreference.com/w/c/language/function_declaration.

PiperOrigin-RevId: 499559101

python/descriptor_pool.c

@@ -45,7 +45,7 @@ typedef struct {
   PyObject* db;  // The DescriptorDatabase underlying this pool.  May be NULL.
 } PyUpb_DescriptorPool;
 
-PyObject* PyUpb_DescriptorPool_GetDefaultPool() {
+PyObject* PyUpb_DescriptorPool_GetDefaultPool(void) {
   PyUpb_ModuleState* s = PyUpb_ModuleState_Get();
   return s->default_pool;
 }

upb/message/accessors.c

@@ -147,7 +147,7 @@ upb_GetExtensionAsBytes_Status upb_MiniTable_GetExtensionAsBytes(
   return kUpb_GetExtensionAsBytes_Ok;
 }
 
-static upb_FindUnknownRet upb_FindUnknownRet_ParseError() {
+static upb_FindUnknownRet upb_FindUnknownRet_ParseError(void) {
   return (upb_FindUnknownRet){.status = kUpb_FindUnknown_ParseError};
 }