include/err: Enforce c linkage for err and warn functions.

This change ensures that xx_func names are not mangled by a c++ compiler
and can be correctly resolved when loaded at runtime.

include/err.h

@@ -32,6 +32,14 @@
  * Pre-processor Definitions
  ****************************************************************************/
 
+#ifdef __cplusplus
+#define EXTERN extern "C"
+extern "C"
+{
+#else
+#define EXTERN extern
+#endif
+
 /* Append _func suffix to avoid the penitential symbol collision */
 
 #define warn   warn_func
@@ -67,4 +75,9 @@ void verr(int status, FAR const char *fmt, va_list ap) printf_like(2, 0);
 void errx(int status, FAR const char *fmt, ...) printf_like(2, 3);
 void verrx(int status, FAR const char *fmt, va_list ap) printf_like(2, 0);
 
+#ifdef __cplusplus
+#undef EXTERN
+}
+#endif
+
 #endif /* __INCLUDE_ERR_H */