raop: Extract encryption related code into a separate file

That makes the raop_client.c code smaller/cleaner and will simplify
addition of more crypto related stuffs like authentication.

src/modules/raop/raop_crypto.h

@@ -0,0 +1,35 @@
+#ifndef fooraopcryptofoo
+#define fooraopcryptofoo
+
+/***
+  This file is part of PulseAudio.
+
+  Copyright 2013 Martin Blanchard
+
+  PulseAudio is free software; you can redistribute it and/or modify
+  it under the terms of the GNU Lesser General Public License as published
+  by the Free Software Foundation; either version 2.1 of the License,
+  or (at your option) any later version.
+
+  PulseAudio is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with PulseAudio; if not, write to the Free Software
+  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
+  USA.
+***/
+
+typedef struct pa_raop_secret pa_raop_secret;
+
+pa_raop_secret* pa_raop_secret_new(void);
+void pa_raop_secret_free(pa_raop_secret *s);
+
+char* pa_raop_secret_get_iv(pa_raop_secret *s);
+char* pa_raop_secret_get_key(pa_raop_secret *s);
+
+int pa_raop_aes_encrypt(pa_raop_secret *s, uint8_t *data, int len);
+
+#endif