samples/subsys/logging/logger: 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>

samples/subsys/logging/logger/src/sample_module.h

@@ -6,10 +6,11 @@
 #ifndef SAMPLE_MODULE_H
 #define SAMPLE_MODULE_H
 
+#include <logging/log.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
-#include <logging/log.h>
 
 #define MODULE_NAME sample_module