Why doesn't gcc require I `#include stdio.h` in my `helloworld.c`

Question

I just took a C aptitude test on http://upwork.com.

One question flummoxed me as the code contained a printf without a corresponding #include stdio.h. Essentially:

int main(){ 
    /* !!! some complicated logic... */

    printf("%d", blah); 
}

And the multiple choice contained the option of a compiler error.

So I wasn't sure if it was a trick question. But gcc compiles int main(){ printf("foo"); } happily on OSX.

Is this guaranteed by the standard?