Fix warning about structure packing when using GCC

Hi,

I am using GCC for AVR32, which does not support ```#pragma pack(push, 1)``` and ```#pragma pack(pop)```: it raises the following warning:
```
[...]/mavlink_types.h:119: warning: #pragma pack(pop[, id], <n>) is not supported on this target
[...]/mavlink_types.h:36: warning: #pragma pack(push[, id], <n>) is not supported on this target
```

I added a macro that defines packed structures according to the compiler:
- with ```__attribute__((packed))``` when using GCC
- with ```__pragma(pack(push, 1))``` ```__pragma(pack(pop))``` if not