Fix compilation failure :

unixd.c: In function ‘ap_unixd_mpm_set_signals’:
unixd.c:579:5: error: implicit declaration of function ‘apr_signal’; did you mean ‘strsignal’? [-Werror=implicit-function-declaration]
     apr_signal(SIGPIPE, SIG_IGN);
     ^~~~~~~~~~
     strsignal

Not sure where it comes from, maybe related to r1812301.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1813116 13f79535-47bb-0310-9956-ffa450edef68