Improve compatibility with C++ consumers

C++ is a little picky about not using keywords for names, or string
constness.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10409

include/os/freebsd/spl/sys/ccompile.h

@@ -294,7 +294,9 @@ typedef long loff_t;
 typedef long rlim64_t;
 typedef int bool_t;
 typedef int enum_t;
+#ifndef __cplusplus
 #define	__init
+#endif
 #define	__exit
 #define	FALSE 0
 #define	TRUE 1