[test] libatexit: leverage __{BEGIN,END}_DECLS

This change converts the longhand form of `extern "C" {` and its
corresponding `}` into `__BEGIN_DECLS` and `__END_DECLS`, respectively.

The new form is much easier to grep for and is a best practice to use in
the FreeBSD tree.

This is meant to be a non-functional change.

MFC after:	1 week

1 files changed, 7 insertions, 4 deletions

diff --git a/lib/libc/tests/stdlib/libatexit/libatexit.cc b/lib/libc/tests/stdlib/libatexit/libatexit.cc
index 7c5a7126d178..5dcbe8a33dc0 100644
--- a/lib/libc/tests/stdlib/libatexit/libatexit.cc
+++ b/lib/libc/tests/stdlib/libatexit/libatexit.cc
@@ -5,15 +5,18 @@
  *
  */
 
+#include <sys/cdefs.h>
 #include <unistd.h>
 
 static int exit_code = -1;
 static bool fatal_atexit;
 
-extern "C" {
-	void set_fatal_atexit(bool);
-	void set_exit_code(int);
-}
+__BEGIN_DECLS
+
+void set_fatal_atexit(bool);
+void set_exit_code(int);
+
+__END_DECLS
 
 void
 set_fatal_atexit(bool fexit)