mb/google/skyrim: Use die_no_apcb

Use die_no_apcb to cause a build error when the APCB or SPD sources are
not found.

TEST=builds with and without matching APCB and SPD sources

Signed-off-by: Fred Reitberger <reitbergerfred@gmail.com>
Change-Id: I62dce2c71061bfc5c01e0344b7dc115a47669140
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73008
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>

src/mainboard/google/skyrim/Makefile.inc

@@ -39,7 +39,9 @@ $(obj)/$(APCB_NAME).gen: $(SPD_SOURCES) \
 			     --mem_type 'lp5'
 else
 $(info SPD sources not found.  Skipping APCB.)
+files_added:: die_no_apcb
 endif
 else
 $(info APCB sources not found.  Skipping APCB.)
+files_added:: die_no_apcb
 endif