coap.h: adding main header file into various source files

After splitting of the platform header part within libcoap some source
files that use functions of the network stack need to include coap.h
(which is including the libcoap.h) to get the platform specific header file
for the network stack.
Sort list of included headers in alphabetical order.

src/async.c

@@ -14,12 +14,11 @@
 #ifndef WITHOUT_ASYNC
 
 #include "coap_config.h"
-
-#include "utlist.h"
-
-#include "mem.h"
-#include "debug.h"
+#include "coap.h"
 #include "async.h"
+#include "debug.h"
+#include "mem.h"
+#include "utlist.h"
 
 coap_async_state_t *
 coap_register_async(coap_context_t *context, coap_address_t *peer,

src/resource.c

@@ -7,12 +7,13 @@
  */
 
 #include "coap_config.h"
-#include "utlist.h"
+#include "coap.h"
+#include "debug.h"
 #include "mem.h"
 #include "net.h"
-#include "debug.h"
 #include "resource.h"
 #include "subscribe.h"
+#include "utlist.h"
 
 #ifdef WITH_LWIP
 /* mem.h is only needed for the string free calls for

src/subscribe.c

@@ -8,6 +8,7 @@
  */
 
 #include "coap_config.h"
+#include "coap.h"
 
 #if defined(HAVE_ASSERT_H) && !defined(assert)
 # include <assert.h>