Bump solana_rbpf

Signed-off-by: Sean Young <sean@mess.org>

Cargo.toml

@@ -54,7 +54,7 @@ wasmi = "0.10"
 rand = "0.7"
 sha2 = "0.10"
 # solana_rbpf makes api changes in patch versions
-solana_rbpf = "=0.2.19"
+solana_rbpf = "=0.2.21"
 byteorder = "1.3"
 assert_cmd = "2.0"
 bincode = "1.3"