Remove user_interface.h from gdbstub includes (#6862)

* Remove user_interface.h from gdbstub includes

Avoid a warning by not including the unneeded "user_interface.h" from
gdbstub.c.  Leftover from original stub code from the SDK, this include
is unnecessary in the Arduino core.

* Add example to ensure gdbstub compiles in CI

Ensure the same user_interface warning doesn't occur in the future by
adding the example shown in the GDB docs to the CI build process.

Author: earlephilhower

libraries/GDBStub/examples/gdbstub_example.ino

@@ -0,0 +1,13 @@
+#include <GDBStub.h>
+
+void setup() {
+  Serial.begin(115200);
+  gdbstub_init();
+  Serial.printf("Starting...\n");
+}
+
+void loop() {
+  static uint32_t cnt = 0;
+  Serial.printf("%d\n", cnt++);
+  delay(100);
+}

libraries/GDBStub/src/internal/gdbstub.c

@@ -67,7 +67,6 @@ OS-less SDK defines. Defines some headers for things that aren't in the include
 the xthal stack frame struct.
 */
 #include "osapi.h"
-#include "user_interface.h"
 
 void _xtos_set_exception_handler(int cause, void (exhandler)(struct XTensa_exception_frame_s *frame));