tests: Plugins are not supported on Windows, yet

For some reason the dlopen() always fails, like this:

 [ RUN      ] test_add_simple
 libyang[2]: Searching for "ietf-yang-types" in "D:\a\oopt-gnpy-libyang\build\libyang\tests\utests".
 libyang[2]: Newer revision than "ietf-yang-types@2013-07-15" not found, using this as the latest revision.
 libyang[2]: Searching for "ietf-inet-types" in "D:\a\oopt-gnpy-libyang\build\libyang\tests\utests".
 libyang[2]: Newer revision than "ietf-inet-types@2013-07-15" not found, using this as the latest revision.
 libyang[0]: Loading "D:/a/oopt-gnpy-libyang/build/libyang/out-plugins/Release/plugin_simple.dll" as a plugin failed ("D:\a\oopt-gnpy-libyang\build\libyang\out-plugins\Release\plugin_simple.dll": The specified module could not be found.).
 [  ERROR   ] --- 0x0 != 0x2
 [   LINE   ] --- D:\a\oopt-gnpy-libyang\oopt-gnpy-libyang\libyang\tests\utests\basic\test_plugins.c:50: error: Failure!
 [  FAILED  ] test_add_simple

Rather than trying to debug this on a platform that I don't really know,
let's declare plugins as not supported on Windows.
diff --git a/src/plugins.c b/src/plugins.c
index 43d70c6..2ea509b 100644
--- a/src/plugins.c
+++ b/src/plugins.c
@@ -494,6 +494,11 @@
 
     LOGERR(NULL, LY_EINVAL, "Plugins are not supported in statically built library.");
     return LY_EINVAL;
+#elif defined (_WIN32)
+    (void)pathname;
+
+    LOGERR(NULL, LY_EINVAL, "Plugins are not (yet) supported on Windows.");
+    return LY_EINVAL;
 #else
     LY_ERR ret = LY_SUCCESS;