Backends: OpenGL3: fixed GL loader to work on Haiku OS which does not support. (#8952)

backends/imgui_impl_opengl3_loader.h

@@ -714,7 +714,11 @@ static void close_libgl(void)
 
 static int is_library_loaded(const char* name, void** lib)
 {
+#if defined(__HAIKU__)
+    *lib = NULL; // no support for RTLD_NOLOAD on Haiku.
+#else
     *lib = dlopen(name, RTLD_LAZY | RTLD_LOCAL | RTLD_NOLOAD);
+#endif
     return *lib != NULL;
 }