spirv-headers: set correct branch name

As of last week, the SPIRV repositories have renamed their master
branches to main:

https://github.com/KhronosGroup/SPIRV-Headers/issues/313



Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2957da09)
Signed-off-by: Steve Sakoman <steve@sakoman.com>