include headers that are missing when --without-ssl is specified

[mirchr]

Add header files to fix the following errors and warnings when --without-ssl is specified during configure.

In file included from idlers.c:2:
conn.h:20: error: expected specifier-qualifier-list before ‘time_t’

conn.c: In function ‘close_conn’:
conn.c:149: warning: implicit declaration of function ‘free’
conn.c:149: warning: incompatible implicit declaration of built-in function ‘free’
conn.c:153: warning: incompatible implicit declaration of built-in function ‘free’

epoll.c:29: warning: implicit declaration of function ‘strerror’

[UlricE]

Merged.