util: include git2/deprecated.h in errors.c

In C99 functions need to be declared before they are defined.
We could just add the declarations before them, but including the
header allows the compiler to warn if they differ.

src/util/errors.c

@@ -376,6 +376,9 @@ void git_error_system_set(int code)
 /* Deprecated error values and functions */
 
 #ifndef GIT_DEPRECATE_HARD
+
+#include "git2/deprecated.h"
+
 const git_error *giterr_last(void)
 {
 	return git_error_last();