Fix use of sizeof without brackets

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/programs/pkey/ecdsa.c b/programs/pkey/ecdsa.c
index 8f9867a..e5d2d44 100644
--- a/programs/pkey/ecdsa.c
+++ b/programs/pkey/ecdsa.c
@@ -77,7 +77,7 @@
     size_t len;
 
     if (mbedtls_ecp_point_write_binary(&key->grp, &key->Q,
-                                       MBEDTLS_ECP_PF_UNCOMPRESSED, &len, buf, sizeof buf) != 0) {
+                                       MBEDTLS_ECP_PF_UNCOMPRESSED, &len, buf, sizeof(buf)) != 0) {
         mbedtls_printf("internal error\n");
         return;
     }