Fix GCC format-signedness warnings

Signed-off-by: Kenneth Soerensen <knnthsrnsn@gmail.com>
diff --git a/programs/pkey/rsa_sign.c b/programs/pkey/rsa_sign.c
index 9bcd7a6..78ab176 100644
--- a/programs/pkey/rsa_sign.c
+++ b/programs/pkey/rsa_sign.c
@@ -129,7 +129,7 @@
     fflush( stdout );
     if( ( ret = mbedtls_rsa_check_privkey( &rsa ) ) != 0 )
     {
-        mbedtls_printf( " failed\n  ! mbedtls_rsa_check_privkey failed with -0x%0x\n", -ret );
+        mbedtls_printf( " failed\n  ! mbedtls_rsa_check_privkey failed with -0x%0x\n", (unsigned int) -ret );
         goto exit;
     }
 
@@ -151,7 +151,7 @@
     if( ( ret = mbedtls_rsa_pkcs1_sign( &rsa, NULL, NULL, MBEDTLS_RSA_PRIVATE, MBEDTLS_MD_SHA256,
                                 20, hash, buf ) ) != 0 )
     {
-        mbedtls_printf( " failed\n  ! mbedtls_rsa_pkcs1_sign returned -0x%0x\n\n", -ret );
+        mbedtls_printf( " failed\n  ! mbedtls_rsa_pkcs1_sign returned -0x%0x\n\n", (unsigned int) -ret );
         goto exit;
     }