headers: use custom LicenseRef- spdx tag for various "public domain" files

There is no spdx tag defined for those versions of "public domain", but we can
add a custom tag, see
https://spdx.github.io/spdx-spec/6-other-licensing-information-detected/.

crc32.h is updated to match crc32.c.

src/basic/MurmurHash2.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LicenseRef-murmurhash2-public-domain */
 //-----------------------------------------------------------------------------
 // MurmurHash2 was written by Austin Appleby, and is placed in the public
 // domain. The author hereby disclaims copyright to this source code.

src/basic/MurmurHash2.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LicenseRef-murmurhash2-public-domain */
 //-----------------------------------------------------------------------------
 // MurmurHash2 was written by Austin Appleby, and is placed in the public
 // domain. The author hereby disclaims copyright to this source code.