Re-order mbedtls_mpi to save a few extra bytes with clang

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index 96cc656..a8422b1 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -203,6 +203,12 @@
  * \brief          MPI structure
  */
 typedef struct mbedtls_mpi {
+    /** Pointer to limbs.
+     *
+     * This may be \c NULL if \c n is 0.
+     */
+    mbedtls_mpi_uint *MBEDTLS_PRIVATE(p);
+
     /** Sign: -1 if the mpi is negative, 1 otherwise.
      *
      * The number 0 must be represented with `s = +1`. Although many library
@@ -221,12 +227,6 @@
 #if MBEDTLS_MPI_MAX_LIMBS > 65535
 #error "MBEDTLS_MPI_MAX_LIMBS > 65535 is not supported"
 #endif
-
-    /** Pointer to limbs.
-     *
-     * This may be \c NULL if \c n is 0.
-     */
-    mbedtls_mpi_uint *MBEDTLS_PRIVATE(p);
 }
 mbedtls_mpi;