AK: Add AK_OS_GNU_HURD :^)

This is defined when building on GNU/Hurd, the GNU operating system with
the Hurd as its kernel (as it was designed originally, before Linux and
GNU/Linux came to be).

Also, define the corresponding part of User-Agent.

AK/Platform.h

@@ -98,6 +98,10 @@
 #    define AK_OS_SOLARIS
 #endif
 
+#if defined(__gnu_hurd__)
+#    define AK_OS_GNU_HURD
+#endif
+
 #if defined(_WIN32) || defined(_WIN64)
 #    define AK_OS_WINDOWS
 #endif

Userland/Libraries/LibWeb/Loader/ResourceLoader.h

@@ -44,6 +44,8 @@ namespace Web {
 #    define OS_STRING "DragonFly"
 #elif defined(AK_OS_SOLARIS)
 #    define OS_STRING "SunOS"
+#elif defined(AK_OS_GNU_HURD)
+#    define OS_STRING "GNU/Hurd"
 #else
 #    error Unknown OS
 #endif