handmade_platform.h could use a stddef.h include

On day 34, the following line was added to handmade_platform.h:

1	typedef size_t memory_index;

This causes a compilation error with g++:

1
2
3

In file included from src/handmade.h:22:0,
                 from src/handmade.cc:9:
src/handmade_platform.h:27:9: error: 'size_t' does not name a type

Including stddef.h alongside the existing stdint.h include fixes that.