Fix configure phpdbg help output (php#12013)

By default phpdbg is enabled (--enable-phpdbg) and user can get info in
the `./configure --help` output if they want to disable it like with the
other configuration options.

Author: petk

sapi/phpdbg/config.m4

@@ -1,7 +1,7 @@
 PHP_ARG_ENABLE([phpdbg],
   [for phpdbg support],
-  [AS_HELP_STRING([--enable-phpdbg],
-    [Build phpdbg])],
+  [AS_HELP_STRING([--disable-phpdbg],
+    [Disable building of phpdbg])],
   [yes],
   [yes])