include/exec: add missing include guard comment

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240620152220.2192768-2-alex.bennee@linaro.org>
diff --git a/include/exec/gdbstub.h b/include/exec/gdbstub.h
index eb14b91..008a921 100644
--- a/include/exec/gdbstub.h
+++ b/include/exec/gdbstub.h
@@ -144,4 +144,4 @@
 /* in gdbstub-xml.c, generated by scripts/feature_to_c.py */
 extern const GDBFeature gdb_static_features[];
 
-#endif
+#endif /* GDBSTUB_H */