Fixed warning about error_at_line

ps/display.c:65:7: warning: implicit declaration of function
‘error_at_line’ [-Wimplicit-function-declaration]

Applied Gentoo patch created by ssuominen

References:
  http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/sys-process/procps/files/procps-3.3.6-error_at_line.patch?annotate=1.1&diff_format=f
  http://www.freelists.org/post/procps/procpsng-337,17

Signed-off-by: Craig Small <csmall@enc.com.au>

ps/display.c

@@ -25,6 +25,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#include <error.h>
 
 #include <sys/sysmacros.h>
 #include <sys/types.h>