fit-dtb.blob.gz: Pass "-n" to gzip

fit-dtb.blob.gz: Pass "-n" to gzip

Sometimes when building a platform such as imx28_btt3 we can get an
error such as:

gzip: fit-dtb.blob: warning: file timestamp out of range for gzip format
make[1]: *** [Makefile:1219: fit-dtb.blob.gz] Error 2
make[1]: *** Deleting file 'fit-dtb.blob.gz'
make: *** [Makefile:186: sub-make] Error 2

This is typically resolved by telling gzip to ignore the timestamp.

Signed-off-by: Tom Rini <trini@konsulko.com>