MBED_STRUCT_STATIC_ASSERT - avoid alignment problem

The `int : 0` bitfield this produced could force integer alignment onto
the structure it was placed in, making a structure that should be 1 byte
be 4 bytes.

Change `int` to `bool` to minimise alignment impact - should be to
nothing.

Alignment/size problem was revealed in a `sizeof` check in an
`Atomic<uint8_t>` test.

platform/mbed_assert.h

@@ -24,6 +24,7 @@
 #ifndef MBED_ASSERT_H
 #define MBED_ASSERT_H
 
+#include <stdbool.h>
 #include "mbed_preprocessor.h"
 #include "mbed_toolchain.h"
 
@@ -124,7 +125,7 @@ do {                                                     \
  *  };
  *  @endcode
  */
-#define MBED_STRUCT_STATIC_ASSERT(expr, msg) int : (expr) ? 0 : -1
+#define MBED_STRUCT_STATIC_ASSERT(expr, msg) bool : (expr) ? 0 : -1
 
 
 #endif