src/lib: Update Makefile to keep build/spd.bin rule private

The rule to make spd.bin that's in src/lib is for the 'generic_spd_bin'
implementation.  It wasn't guarded though, so it was generating a build
warning for any other platform that generated an spd.bin file.

Sample warning that this fixes:
src/mainboard/gizmosphere/gizmo/Makefile.inc:42:
warning: overriding recipe for target 'build/spd.bin'
src/lib/Makefile.inc:298: warning: ignoring old recipe for target
'build/spd.bin'

Change-Id: Iadd6743f8ae476969bf36f99b918f04c04172d1d
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: https://review.coreboot.org/18261
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/src/lib/Makefile.inc b/src/lib/Makefile.inc
index 9a9ddc8..38b3c1228 100644
--- a/src/lib/Makefile.inc
+++ b/src/lib/Makefile.inc
@@ -287,6 +287,7 @@
 
 endif # CONFIG_RAMSTAGE_LIBHWBASE
 
+ifeq ($(CONFIG_GENERIC_SPD_BIN),y)
 romstage-$(CONFIG_GENERIC_SPD_BIN) += spd_bin.c
 
 LIB_SPD_BIN = $(obj)/spd.bin
@@ -304,3 +305,4 @@
 cbfs-files-$(CONFIG_GENERIC_SPD_BIN) += spd.bin
 spd.bin-file := $(LIB_SPD_BIN)
 spd.bin-type := spd
+endif