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/ptp_clock.h

@@ -92,10 +92,10 @@ static inline int ptp_clock_rate_adjust(struct device *dev, float rate)
 	return api->rate_adjust(dev, rate);
 }
 
-#include <syscalls/ptp_clock.h>
-
 #ifdef __cplusplus
 }
 #endif
 
+#include <syscalls/ptp_clock.h>
+
 #endif /* ZEPHYR_INCLUDE_PTP_CLOCK_H_ */