Silence compiler warning in example program

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/programs/psa/aead_cipher_psa.c b/programs/psa/aead_cipher_psa.c
index 301701c..ef73e7a 100644
--- a/programs/psa/aead_cipher_psa.c
+++ b/programs/psa/aead_cipher_psa.c
@@ -88,7 +88,7 @@
     do {                    \
         ret = code;         \
         if( ret != 0 ) {    \
-            printf( "%s:%03d: ret = -0x%04x\n", __func__, __LINE__, -ret ); \
+            printf( "%s:%03d: ret = -0x%04x\n", __func__, __LINE__, (unsigned) -ret ); \
             goto exit;      \
         }                   \
     } while( 0 )