fix utils.cpp failed to compile.

==> Starting build()...
[CXX] main.cpp
[CXX] md5.cpp
[CXX] utils.cpp
utils.cpp: In function ‘std::string md5sum(const char*, ...)’:
utils.cpp:31:18: error: ‘va_start’ was not declared in this scope
  va_start(ap, fmt);
                  ^
utils.cpp:34:12: error: ‘va_end’ was not declared in this scope
   va_end(ap);
            ^
utils.cpp:37:11: error: ‘va_end’ was not declared in this scope
  va_end(ap);
           ^
utils.cpp: In function ‘void eprintf(const char*, ...)’:
utils.cpp:77:18: error: ‘va_start’ was not declared in this scope
  va_start(ap, fmt);
                  ^
utils.cpp:80:12: error: ‘va_end’ was not declared in this scope
   va_end(ap);
            ^
utils.cpp:83:11: error: ‘va_end’ was not declared in this scope
  va_end(ap);
           ^
utils.cpp: In function ‘void iprintf(const char*, ...)’:
utils.cpp:99:18: error: ‘va_start’ was not declared in this scope
  va_start(ap, fmt);
                  ^
utils.cpp:102:12: error: ‘va_end’ was not declared in this scope
   va_end(ap);
            ^
utils.cpp:105:11: error: ‘va_end’ was not declared in this scope
  va_end(ap);
           ^
Makefile:19: recipe for target 'utils.o' failed
make: *** [utils.o] Error 1