fd_util: check HAVE_INOTIFY_INIT in header

Don't provide the prototype if inotify_init() was not detected.

src/fd_util.h

@@ -122,6 +122,8 @@ recvmsg_cloexec(int sockfd, struct msghdr *msg, int flags);
 
 #endif
 
+#ifdef HAVE_INOTIFY_INIT
+
 /**
  * Wrapper for inotify_init(), which sets the CLOEXEC flag (atomically
  * if supported by the OS).
@@ -130,3 +132,5 @@ int
 inotify_init_cloexec(void);
 
 #endif
+
+#endif