build: fix missing `$$(@D)/` from `sp_gen.mk`

This target can currently fail as the `$(BUILD_PLAT)` target no longer exists, as it is now `$(BUILD_PLAT)/` (with an explicit trailing slash).

Change-Id: I9172b2f51f6e02e6369f62468ea63a64ec0f6dd1
Signed-off-by: Chris Kay <chris.kay@arm.com>