Add GPL headers to cubic benchmark.

The source for this benchmark is in the public domain, release the BEEBS
version under GPL.

ChangeLog:

	* src/cubic/basicmath_small.c: Add GPL header.
	* src/cubic/cubic.c: Add GPL header.
	* src/cubic/pi.h: Add GPL header.
	* src/cubic/snipmath.h: Add GPL header.
	* src/cubic/sniptype.h: Add GPL header.