The "common.h" should be included before "config.h".

When building libgit2 for ia32 architecture on a x64 machine under Windows, including "config.h" without a "common.h" would result in the following error:

```
C:\Program Files\Microsoft SDKs\Windows\v7.1\include\winbase.h(2288): error C2373: 'InterlockedIncrement' : redefinition; different type modifiers [C:\cygwin\home\zcbenz\codes\git-utils\build\libgit2.vcxproj]
C:\Program Files\Microsoft SDKs\Windows\v7.1\include\winbase.h(2295): error C2373: 'InterlockedDecrement' : redefinition; different type modifiers [C:\cygwin\home\zcbenz\codes\git-utils\build\libgit2.vcxproj]
C:\Program Files\Microsoft SDKs\Windows\v7.1\include\winbase.h(2303): error C2373: 'InterlockedExchange' : redefinition; different type modifiers [C:\cygwin\home\zcbenz\codes\git-utils\build\libgit2.vcxproj]
C:\Program Files\Microsoft SDKs\Windows\v7.1\include\winbase.h(2314): error C2373: 'InterlockedExchangeAdd' : redefinition; different type modifiers [C:\cygwin\home\zcbenz\codes\git-utils\build\libgit2.vcxproj]
```