lib/libc: rearrange for standard use of extern "C"

Consistently place C++ use of extern "C" after all include directives,
within the negative branch of _ASMLANGUAGE if used.

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>

lib/libc/minimal/include/time.h

@@ -11,6 +11,7 @@
 #include <stdint.h>
 #include <sys/_types.h>
 #include <bits/restrict.h>
+#include <sys/_timespec.h>
 
 /* Minimal time.h to fulfill the requirements of certain libraries
  * like mbedTLS and to support time APIs.
@@ -42,8 +43,6 @@ typedef _TIME_T_ time_t;
 typedef _SUSECONDS_T_ suseconds_t;
 #endif
 
-#include <sys/_timespec.h>
-
 /*
  * Conversion between civil time and UNIX time.  The companion
  * localtime() and inverse mktime() are not provided here since they