Correct dangerous typo in include/polarssl/ssl.h

The definition of SSL_MAC_ADD depends on the presence of the
configuration option POLARSSL_ARC4_C, which was misspelled as
POLARSSL_RC4_C in ssl.h, leading to a too small buffer and
subsequently to a buffer overflow during record processing.
This commit fixes the typo.
diff --git a/include/polarssl/ssl.h b/include/polarssl/ssl.h
index 9a3fb8a..32c07c2 100644
--- a/include/polarssl/ssl.h
+++ b/include/polarssl/ssl.h
@@ -303,7 +303,7 @@
 #define SSL_COMPRESSION_ADD             0
 #endif
 
-#if defined(POLARSSL_RC4_C) || defined(POLARSSL_CIPHER_MODE_CBC)
+#if defined(POLARSSL_ARC4_C) || defined(POLARSSL_CIPHER_MODE_CBC)
 /* Ciphersuites using HMAC */
 #if defined(POLARSSL_SHA512_C)
 #define SSL_MAC_ADD                 48  /* SHA-384 used for HMAC */