fixed redefinition of typedef ‘lwan_t’

lwan.h:75: error: redefinition of typedef ‘lwan_t’
lwan-status.h:23: note: previous declaration of ‘lwan_t’ was here
make[2]: *** [CMakeFiles/lwan-common.dir/int-to-str.c.o] Error 1
make[1]: *** [CMakeFiles/lwan-common.dir/all] Error 2