Fix GNU/Hurd build

It has the usual Unix filesystem operations.

modules/core/include/opencv2/core/utils/filesystem.private.hpp

@@ -12,7 +12,8 @@
 #  elif defined WINRT || defined _WIN32_WCE
      /* not supported */
 #  elif defined __ANDROID__ || defined __linux__ || defined _WIN32 || \
-        defined __FreeBSD__ || defined __bsdi__ || defined __HAIKU__
+        defined __FreeBSD__ || defined __bsdi__ || defined __HAIKU__ || \
+        defined __GNU__
 #      define OPENCV_HAVE_FILESYSTEM_SUPPORT 1
 #  elif defined(__APPLE__)
 #    include <TargetConditionals.h>

modules/core/src/utils/filesystem.cpp

@@ -34,7 +34,7 @@
 #include <errno.h>
 #include <io.h>
 #include <stdio.h>
-#elif defined __linux__ || defined __APPLE__ || defined __HAIKU__ || defined __FreeBSD__
+#elif defined __linux__ || defined __APPLE__ || defined __HAIKU__ || defined __FreeBSD__ || defined __GNU__
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
@@ -343,7 +343,7 @@ private:
     Impl& operator=(const Impl&); // disabled
 };
 
-#elif defined __linux__ || defined __APPLE__ || defined __HAIKU__ || defined __FreeBSD__
+#elif defined __linux__ || defined __APPLE__ || defined __HAIKU__ || defined __FreeBSD__ || defined __GNU__
 
 struct FileLock::Impl
 {
@@ -457,7 +457,7 @@ cv::String getCacheDirectory(const char* sub_directory_name, const char* configu
             default_cache_path = "/tmp/";
             CV_LOG_WARNING(NULL, "Using world accessible cache directory. This may be not secure: " << default_cache_path);
         }
-#elif defined __linux__ || defined __HAIKU__ || defined __FreeBSD__
+#elif defined __linux__ || defined __HAIKU__ || defined __FreeBSD__ || defined __GNU__
         // https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html
         if (default_cache_path.empty())
         {