common: Include stat.h in include/git2/common.h instead of src/common.h

00582bcb introduced a change to git_blob_create_fromfile() that required
the caller to pass a stat struct. This means that we need to include
stat.h higher in the hierarchy of includes.

include/git2/common.h

@@ -28,6 +28,7 @@
 #include "thread-utils.h"
 #include <time.h>
 #include <stdlib.h>
+#include <sys/stat.h>
 
 #ifdef __cplusplus
 # define GIT_BEGIN_DECL  extern "C" {

src/common.h

@@ -22,7 +22,6 @@
 #include <string.h>
 
 #include <sys/types.h>
-#include <sys/stat.h>
 
 #ifdef GIT_WIN32