import if you have to
As noted in the very first answer, some compilers (notably, GCC and clang) let you get away with omitting #includes for standard library functions.
Even if you can't just remove the #include, there might be other ways to avoid it, but that's not always practical or particularly golfy.
In the remaining cases, you can use #import<header file> instead of #include<header file> to save a byte. This is a GNU extension and it is considered deprecated, but it works at least in gcc 4.8, gcc 5.1, and clang 3.7.
