[easytag] Add unistd.h include for SEEK_SET under MinGW

[David King <davidk src gnome org>]

commit ec35ca5930a91a50870a3988742dcd3a779a45c8
Author: David King <amigadave amigadave com>
Date:   Wed Dec 31 19:03:37 2014 +0000

    Add unistd.h include for SEEK_SET under MinGW

 src/tags/flac_private.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/src/tags/flac_private.c b/src/tags/flac_private.c
index 16494a0..bdda107 100644
--- a/src/tags/flac_private.c
+++ b/src/tags/flac_private.c
@@ -19,6 +19,7 @@
 #include "flac_private.h"
 
 #include <errno.h>
+#include <unistd.h>
 
 size_t
 et_flac_read_func (void *ptr,