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>
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