config_file: add missing include for `git_config_backend`

The config_file.h header provides some inline declarations accessing the
`git_config_backend`, but misses its declaration. Add the missing
include for "git2/sys/config.h" to add it.

src/config_file.h

@@ -7,6 +7,7 @@
 #ifndef INCLUDE_config_file_h__
 #define INCLUDE_config_file_h__
 
+#include "git2/sys/config.h"
 #include "git2/config.h"
 
 GIT_INLINE(int) git_config_file_open(git_config_backend *cfg, unsigned int level)