popen: add const qualifier to popen_write_timeout

The buffer is for reading, we're not intend to change it.

Part of #4031

Acked-by: Cyrill Gorcunov <gorcunov@gmail.com>

src/lib/core/popen.c

@@ -280,7 +280,7 @@ stdX_str(unsigned int index)
  * Write data to the child stdin.
  */
 int
-popen_write_timeout(struct popen_handle *handle, void *buf,
+popen_write_timeout(struct popen_handle *handle, const void *buf,
 		    size_t count, unsigned int flags,
 		    ev_tstamp timeout)
 {

src/lib/core/popen.h

@@ -154,7 +154,7 @@ extern const char *
 popen_command(struct popen_handle *handle);
 
 extern int
-popen_write_timeout(struct popen_handle *handle, void *buf,
+popen_write_timeout(struct popen_handle *handle, const void *buf,
 		    size_t count, unsigned int flags,
 		    ev_tstamp timeout);