Include time.h for ios

[davidsiaw]

An as-of-yet incomplete project of mine embedding mruby in iOS has revealed that struct timeval on recent versions of Darwin iOS is defined in sys/time.h.

This should have a minimal impact on other platforms.

If you wish to have a test for this I will be able to produce one in time. Currently I use a very hacked up xcode project file that includes a set of C files generated by mrbgems, and the project file is not fit for general consumption because it only includes a very specific set of mrbgems.

Once I have a working project file generation tool I will share it and this problem will become obvious, or if you have a minimal project using iOS you can see this issue.

I may be doing it wrong and could have been a build setting away from making mruby compile on iOS without this change, and would gladly take advice on how to proceed.