PHPC-2501: Conditionally define MONGOC_CYRUS_PLUGIN_PATH_PREFIX on Windows

This was missed in 6a2e15a when bumping to libmongoc 1.26.2.

Define the constant as null to satisfy compilation. It will not actually be used for path prefix checking.

Author: jmikola

config.w32

@@ -239,6 +239,10 @@ if (PHP_MONGODB != "no") {
       CHECK_HEADER_ADD_INCLUDE("sasl/sasl.h", "CFLAGS_MONGODB")) {
     mongoc_opts.MONGOC_ENABLE_SASL = 1;
     mongoc_opts.MONGOC_ENABLE_SASL_CYRUS = 1;
+
+    // Referenced by _mongoc_cyrus_verifyfile_cb in mongoc-cyrus.c on Windows
+    ADD_FLAG("CFLAGS_MONGODB", "/D MONGOC_CYRUS_PLUGIN_PATH_PREFIX=0");
+
     if (CHECK_FUNC_IN_HEADER("sasl/sasl.h", "sasl_client_done")) {
       mongoc_opts.MONGOC_HAVE_SASL_CLIENT_DONE = 1;
     }