[build] Add missing "const" qualifiers

This fixes "initialization discards 'const' qualifier from pointer
target type" warnings with GCC 5.1.0.

Signed-off-by: Christian Hesse <mail@eworm.de>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tls.c

 			uint8_t length[3];
 			uint8_t payload[0];
 		} __attribute__ (( packed )) *handshake = data;
-		void *payload = &handshake->payload;
+		const void *payload = &handshake->payload;
 		size_t payload_len = tls_uint24 ( handshake->length );
-		void *next = ( payload + payload_len );
+		const void *next = ( payload + payload_len );
 
 		/* Sanity check */
 		if ( next > end ) {