soc/arm/arm: rearrange for standard use of extern "C"

Move the tail include file outside the 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>