net: Move include files outside of extern "C" block

This is related to findings in #17997 and changes network related
header files to have include files outside of extern "C" { } block.

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: Jukka Rissanen <jukka.rissanen@linux.intel.com>

include/net/net_core.h

@@ -13,12 +13,12 @@
 #ifndef ZEPHYR_INCLUDE_NET_NET_CORE_H_
 #define ZEPHYR_INCLUDE_NET_NET_CORE_H_
 
+#include <stdbool.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#include <stdbool.h>
-
 /**
  * @brief Networking
  * @defgroup networking Networking