Bignum: make const placement consistent

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/library/bignum_core.c b/library/bignum_core.c
index a05df99..431921a 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -185,7 +185,7 @@
                               const unsigned char *buf,
                               size_t buflen )
 {
-    size_t const limbs = CHARS_TO_LIMBS( buflen );
+    const size_t limbs = CHARS_TO_LIMBS( buflen );
 
     if( nx < limbs )
         return( MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL );