Moving alignment information in the nsapi_msghdr_t

Macro MBED_ALIGN expands in C to _Alignas which can't be used in the
type declaration. This patch moves it to the first type definition
which makes this code compile properly in CPP and C.

connectivity/netsocket/include/netsocket/nsapi_types.h

@@ -411,8 +411,11 @@ typedef struct nsapi_stagger_req {
 
 /** nsapi_msghdr
  */
-typedef struct MBED_ALIGN(max_align_t) nsapi_msghdr {
-    nsapi_size_t len;    /* Data byte count, including header */
+typedef struct nsapi_msghdr {
+    /* In C alignment can't be specified in the type declaration.
+     * Specifying it in the first member definition will affect whole structure.
+     */
+    MBED_ALIGN(max_align_t) nsapi_size_t len;    /* Data byte count, including header */
     int          level;  /* Originating protocol */
     int          type;   /* Protocol-specific type */
 } nsapi_msghdr_t;