Microcode: add dependency to header files

When using microcode header and updating the header (due to a new
release of microcode during early development) the build system
doesn't detect the header change. This commit fixes this by adding
the appropriate dependency.

Change-Id: I4211a3e39f67da727ef7cddbbee6d8c4718dee4a
Signed-off-by: Julien Viard de Galbert <jviarddegalbert@online.net>
Reviewed-on: https://review.coreboot.org/22307
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/src/cpu/Makefile.inc b/src/cpu/Makefile.inc
index f74e354..46e5419 100644
--- a/src/cpu/Makefile.inc
+++ b/src/cpu/Makefile.inc
@@ -25,7 +25,7 @@
 cbfs-files-y  += cpu_microcode_blob.bin
 cpu_microcode_blob.bin-file = $(objgenerated)/microcode.bin
 
-$(objgenerated)/microcode.bin:
+$(objgenerated)/microcode.bin: $(call strip_quotes,$(CONFIG_CPU_MICROCODE_HEADER_FILES))
 	echo "   util/scripts/ucode_h_to_bin.sh $(objgenerated)/microcode.bin \"$(CONFIG_CPU_MICROCODE_HEADER_FILES)\""
 	util/scripts/ucode_h_to_bin.sh $(objgenerated)/microcode.bin $(CONFIG_CPU_MICROCODE_HEADER_FILES)
 endif