Fix some build issues for dexter:

Fix some build issues for dexter:

. dexter/scoreboard.c needed apr_strings.h to get the right
  function prototypes

. main/mpm_common.c needed to recognize that we were building
  for dexter; otherwise, no ap_reclaim_child_processes() was
  compiled and linking failed

(It would be nice to standardize on which preprocessor symbols
are checked for...  mpmname_MPM seems nice enough.  Didn't the
check for symbol mpmname (no "_MPM") come with mpmt.c?)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85899 13f79535-47bb-0310-9956-ffa450edef68