Related to #17997, for the POSIX arch:
* Remove some unnecessary extern "C" and ifdef blocks
* Move an include out of one of these blocks
* Add a missing extern "C" block
Background:
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: Alberto Escolar Piedras <alpi@oticon.com>
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