Add GitPython's standard license header comments to oss-fuzz scripts

These files are already BSD-3-Clause even without the headers, but
adding these comments and the `LICENSE-BSD` symlink to the root level
`LICENSE` file are helpful to reinforce that there are only two
particular files in the `fuzzing/` that are not under BSD-3-Clause.

See:
#1901 (comment)

fuzzing/LICENSE-BSD

@@ -0,0 +1 @@
+../LICENSE

fuzzing/oss-fuzz-scripts/build.sh

@@ -1,4 +1,7 @@
 # shellcheck shell=bash
+#
+# This file is part of GitPython and is released under the
+# 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
 
 set -euo pipefail
 

fuzzing/oss-fuzz-scripts/container-environment-bootstrap.sh

@@ -1,4 +1,8 @@
 #!/usr/bin/env bash
+#
+# This file is part of GitPython and is released under the
+# 3-Clause BSD License: https://opensource.org/license/bsd-3-clause/
+
 set -euo pipefail
 
 #################