M2AP: Align 8 bits in liblte_m2ap_unpack_protocolie_singlecontainer

Alignment with more then 8 bits is unusual for APER encoding.

It seems clangs uses different packing for the LIBLTE_BIT_MSG_STRUCT
struct then gcc and puts more bytes between N_bits and header.

typedef struct {
  uint32 N_bits;
  uint8  header[LIBLTE_MSG_HEADER_OFFSET];
  uint8  msg[LIBLTE_MAX_MSG_SIZE_BITS];
} LIBLTE_BIT_MSG_STRUCT __attribute__((aligned(8)));

Fixes a failure in srslte_asn1_m2ap_test when compiled with clang
272: Test command: lib/test/asn1/srslte_asn1_m2ap_test
272: Test timeout computed to be: 1500
272: 03:47:13.189195 [M2AP] [I] M2 Setup Request original message
272:              0000: 00 05 00 37 00 00 03 00 0d 00 08 00 00 f1 10 00
272:              0010: 1a 2d 00 00 0e 40 0a 03 80 65 6e 62 31 61 32 64
272:              0020: 30 00 0f 00 16 00 00 10 00 11 00 00 f1 10 1a 2d
272:              0030: 00 10 27 10 01 02 00 01 02 00 02
272: [m2_setup_request_test][Line 95]: FAIL at (plmn_id[0] & 0x0F) == 0 && (plmn_id[0] & 0xF0) >> 4 == 0 && (plmn_id[1] & 0x0F) == 1
1/1 Test #272: srslte_asn1_m2ap_test ............***Failed    0.00 sec

lib/src/asn1/liblte_m2ap.cc

@@ -198,7 +198,7 @@ LIBLTE_ERROR_ENUM liblte_m2ap_unpack_protocolie_singlecontainer(uint8_t**
 
     // Enum - ie->criticality
     ie->criticality = (LIBLTE_M2AP_CRITICALITY_ENUM)liblte_bits_2_value(ptr, 2);
-    liblte_align_up(ptr, 16);
+    liblte_align_up(ptr, 8);
 
     err = LIBLTE_SUCCESS;
   }