include/bluetooth: rearrange for standard use of extern "C"

Consistently place C++ use of extern "C" after all include directives,
within the negative branch of _ASMLANGUAGE if used.

Background from issue #17997:

Declarations that use C linkage should be placed within extern "C"
so the language linkage is correct when the header is included by
a C++ compiler.

Similarly #include directives should be outside the extern "C" to
ensure the language-specific default linkage is applied to any
declarations provided by the included header.

See: https://en.cppreference.com/w/cpp/language/language_linkage
Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>

include/bluetooth/conn.h

@@ -17,15 +17,15 @@
  * @{
  */
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #include <stdbool.h>
 
 #include <bluetooth/bluetooth.h>
 #include <bluetooth/hci.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /** Opaque type representing a connection to a remote device */
 struct bt_conn;