Fix compilation warning.
Fix compilation warning.

* test/test_context.c
  (): Include "serf_private.h" for serf_log().

google-code-export/serf,SirJson/serf,alex1818/serf,alex1818/serf,soufianeEL/serf,google-code-export/serf,SirJson/serf,stoneCC/serf,zhengcq315/serf,SirJson/serf,yesoce/serf,alex1818/serf,x-clone/serf,pengdu/serf,stoneCC/serf,yesoce/serf,soufianeEL/serf,pengdu/serf,pengdu/serf,zhupan/serf,SirJson/serf,zhengcq315/serf,zhupan/serf,KyleSanderson/serf,google-code-export/serf,soufianeEL/serf,KyleSanderson/serf,pengdu/serf,KyleSanderson/serf,lazysoez/serf,zhengcq315/serf,yesoce/serf,x-clone/serf,x-clone/serf,yesoce/serf,KyleSanderson/serf,stoneCC/serf,zhupan/serf,lazysoez/serf,lazysoez/serf,stoneCC/serf,x-clone/serf,lazysoez/serf,zhengcq315/serf,alex1818/serf,zhupan/serf,google-code-export/serf,soufianeEL/serf