execute: Update comment

[Bruno Haible]

pipe.h was renamed to spawn-pipe.h in 2010.


2018-04-07 Bruno Haible <***@clisp.org>

execute: Update comment.
* lib/execute.h (execute): Refer to spawn-pipe.h, not pipe.h.

diff --git a/lib/execute.h b/lib/execute.h
index dfa39a3..f8f92b0 100644
--- a/lib/execute.h
+++ b/lib/execute.h
@@ -33,7 +33,7 @@
the subprocess (if supported by the platform: not on native Windows
platforms), otherwise 0.
It is recommended that no signal is blocked or ignored while execute()
- is called. See pipe.h for the reason. */
+ is called. See spawn-pipe.h for the reason. */
extern int execute (const char *progname,
const char *prog_path, char **prog_argv,
bool ignore_sigpipe,