build: fix compiler issues with missing endian.h include

This commit brings in 2 missing endian.h includes to support the be16toh() call

Without this patch, the following compiler errors occur:

In file included from ./common/memutils.h:17,
                 from common/authenticator/authenticator.c:31:
common/authenticator/authenticator.c: In function ‘auth_send_eapol’:
common/authenticator/authenticator.c:447:68: warning: implicit declaration of function ‘be16toh’ [-Wimplicit-function-declaration]
  447 |               val_to_str(hdr->packet_type, eapol_frames, "[UNK]"), be16toh(hdr->packet_body_length));
      |                                                                    ^~~~~~~

common/authenticator/authenticator_eap.c: In function ‘auth_eap_recv’:
common/authenticator/authenticator_eap.c:325:9: warning: implicit declaration of function ‘be16toh’ [-Wimplicit-function-declaration]
  325 |     if (be16toh(eap->length) > buf_len) {
      |         ^~~~~~~

Per man endian(3), the be16toh() #define is in <endian.h>

Origin: #33

Author: Scott Kilau

common/authenticator/authenticator.c

@@ -15,6 +15,7 @@
 #define _DEFAULT_SOURCE
 #include <netinet/in.h>
 #include <inttypes.h>
+#include <endian.h>
 #include <errno.h>
 #include <string.h>
 

common/authenticator/authenticator_eap.c

@@ -11,6 +11,8 @@
  *
  * [1]: https://www.silabs.com/about-us/legal/master-software-license-agreement
  */
+#include <endian.h>
+
 #include "common/authenticator/authenticator.h"
 #include "common/authenticator/authenticator_radius.h"
 #include "common/authenticator/authenticator_key.h"