configure.ac: register main/io as a build directory

PHP_ADD_SOURCES([main/io], ...) adds the new I/O subsystem sources
(php_io.c, php_io_copy_*.c) to the build, but main/io was never added
to the PHP_ADD_BUILD_DIR list. Since only directories in that list get
pre-created for out-of-tree (VPATH) builds, main/io/ never existed in
such build trees, causing:

  fatal error: opening dependency file main/io/php_io_copy_freebsd.dep:
  No such file or directory

for every file in main/io/ when building outside the source tree
(e.g. Debian's separate fpm-build/cli-build directories).

Add main/io to PHP_ADD_BUILD_DIR, alongside the other source
directories (main, main/poll, main/streams, TSRM, Zend, ...).

Author: mati75

configure.ac

@@ -1834,6 +1834,7 @@ AC_DEFINE([HAVE_BUILD_DEFS_H], [1],
 
 PHP_ADD_BUILD_DIR([
   main
+  main/io
   main/poll
   main/streams
   scripts