[failing-alloc] Make it compile as C++ as well

src/failing-alloc.c

@@ -25,6 +25,10 @@
 #include <stdlib.h>
 #include <stdio.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 int alloc_state = 0;
 
 __attribute__((no_sanitize("integer")))
@@ -55,3 +59,7 @@ void  hb_free_impl (void *ptr)
 {
   return free (ptr);
 }
+
+#ifdef __cplusplus
+}
+#endif