Fix warnings about unreachable code

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/bignum_core.c b/library/bignum_core.c
index 48b640b..85dca55 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -99,6 +99,7 @@
         /* Nothing to do on bigendian systems. */
         return a;
     } else {
+MBEDTLS_IGNORE_UNREACHABLE_BEGIN
         switch (sizeof(mbedtls_mpi_uint)) {
             case 4:
                 return (mbedtls_mpi_uint) MBEDTLS_BSWAP32((uint32_t) a);
@@ -109,6 +110,7 @@
         /* Fall back to C-based reordering if we don't know the byte order
          * or we couldn't use a compiler-specific builtin. */
         return mpi_bigendian_to_host_c(a);
+MBEDTLS_IGNORE_UNREACHABLE_END
     }
 }