Add missing license headers

Some implementation files were missing the license headers. This commit
adds them.

src/attr.c

+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
 #include "common.h"
 #include "repository.h"
 #include "sysdir.h"

src/attr_file.c

+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
 #include "common.h"
 #include "repository.h"
 #include "filebuf.h"

src/attrcache.c

+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
 #include "common.h"
 #include "repository.h"
 #include "attr_file.h"

src/ignore.c

+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
 #include "git2/ignore.h"
 #include "common.h"
 #include "ignore.h"

src/patch.c

+/*
+* Copyright (C) the libgit2 contributors. All rights reserved.
+*
+* This file is part of libgit2, distributed under the GNU GPL v2 with
+* a Linking Exception. For full terms see the included COPYING file.
+*/
+
 #include "git2/patch.h"
 #include "diff.h"
 #include "patch.h"

src/pool.c

+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
 #include "pool.h"
 #include "posix.h"
 #ifndef GIT_WIN32

src/sortedcache.c

+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
 #include "sortedcache.h"
 
 int git_sortedcache_new(