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

This reverts commit df1c98ab6d6171ed63729195bd190b54b67fe530.

As 8a27b6b reverts the exposition of struct stat to the external API, we
do not need - indeed, do not want - struct stat to be in the outer
include layer.

include/git2/common.h

@@ -28,7 +28,6 @@
 #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

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