Include <unistd.h> in src/util.h
On FreeBSD 11-CURRENT this fixes the compilation failure:
In file included from ./src/proxy-bio.h:16,
from src/proxy-bio.c:38:
./src/util.h:46: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'wait_with_timeout'
./src/util.h:50: warning: 'struct timeval' declared inside parameter list
./src/util.h:50: warning: its scope is only this definition or declaration, which is probably not what you want
./src/util.h:51: warning: 'struct timeval' declared inside parameter list
./src/util.h:57: warning: 'struct timeval' declared inside parameter list
*** Error code 1
1 file changed