Fix broken tree (sp_error() undeclared).

src/box/engine_sophia.cc

@@ -45,6 +45,8 @@
 #include <dirent.h>
 #include <errno.h>
 
+extern "C" void *sp_error(void *o, ...);
+
 struct Sophia: public Engine {
 	Sophia(EngineFactory*);
 };

src/box/sophia_index.cc

@@ -42,6 +42,8 @@
 #include <stdio.h>
 #include <inttypes.h>
 
+extern "C" void *sp_error(void *o, ...);
+
 static inline void
 sophia_delete(void *db, struct key_def *key_def, struct tuple *tuple)
 {