include: rearrange for standard use of extern "C" in various headers

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

The inclusion of the generated syscall files is placed outside the
extern "C" block as the generated file has its own extern "C" block.

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/net/can.h

@@ -13,15 +13,16 @@
 #ifndef ZEPHYR_INCLUDE_NET_CAN_H_
 #define ZEPHYR_INCLUDE_NET_CAN_H_
 
-#ifdef __cplusplus
-extern "C" {
-#endif
-
 #include <zephyr/types.h>
 #include <net/net_ip.h>
 #include <net/net_if.h>
 #include <can.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
 /**
  * @brief IPv6 over CAN library
  * @defgroup net_can Network Core Library