dfp.m4: Add missing license header

It seems the license header was omitted when dfp.m4 was originally
contributed in 2010 (commit 3c39bca6, r0-102573 or svn r163815.

This copies the license from libdecnumber/configure.ac since dfp.m4
was originally extracted from that file.

2022-04-29  Christophe Lyon  <christophe.lyon@arm.com>

	config/
	* dfp.m4: Add license header.