Code quality: Fix header guards consistency

Adds `header_guards.sh` bash script, used in CI to validate future
changes. Can be run locally to fix invalid header guards.

modules/mbedtls/stream_peer_mbedtls.h

@@ -28,8 +28,8 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 
-#ifndef STREAM_PEER_OPEN_SSL_H
-#define STREAM_PEER_OPEN_SSL_H
+#ifndef STREAM_PEER_MBEDTLS_H
+#define STREAM_PEER_MBEDTLS_H
 
 #include "core/io/stream_peer_ssl.h"
 #include "ssl_context_mbedtls.h"
@@ -76,4 +76,4 @@ public:
 	~StreamPeerMbedTLS();
 };
 
-#endif // STREAM_PEER_SSL_H
+#endif // STREAM_PEER_MBEDTLS_H