httpdate: minor size reduction in DSO

Extra pointers waste space in the DSO.  Normally I wouldn't
care, but the string lengths are identical and this code
already made it into another project in this form.

size(1) output:

         text      data     bss     dec     hex  filename
before: 42881      2040     336   45257    b0c9  unicorn_http.so
 after: 42499      1888     336   44723    aeb3  unicorn_http.so

ref: http://www.akkadia.org/drepper/dsohowto.pdf