contrib/libxo: fix API header files inclusions in C++ source files

C++ source files need `extern "C"` to disable C++ name mangling.

MFC after:	1 week
Reviewed by:	aokblast (previous version), phil, imp (previous version)
Differential Revision:	https://reviews.freebsd.org/D47930

contrib/libxo/libxo/xo.h

@@ -27,6 +27,10 @@
 #include <stdlib.h>
 #include <errno.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif /* __cplusplus */
+
 #ifdef __dead2
 #define NORETURN __dead2
 #else
@@ -699,4 +703,8 @@ xo_retain_clear_all (void);
 void
 xo_retain_clear (const char *fmt);
 
+#ifdef __cplusplus
+}
+#endif /* __cplusplus */
+
 #endif /* INCLUDE_XO_H */

contrib/libxo/libxo/xo_encoder.h

@@ -20,6 +20,10 @@
 
 #include <string.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif /* __cplusplus */
+
 /*
  * Expose libxo's memory allocation functions
  */
@@ -167,4 +171,8 @@ xo_encoder_op_name (xo_encoder_op_t op);
 void
 xo_failure (xo_handle_t *xop, const char *fmt, ...);
 
+#ifdef __cplusplus
+}
+#endif /* __cplusplus */
+
 #endif /* XO_ENCODER_H */