Makefile.proof: Add all source files to the dependencies

Change-Id: Ic1d19e4516bb20b9796ac02baee099e9df472ab5
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/17021
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/Makefile.proof b/Makefile.proof
index 91b98c9..8b06cdc 100644
--- a/Makefile.proof
+++ b/Makefile.proof
@@ -62,7 +62,7 @@
 	echo '   for Object_Dir use external ("obj", "build");'	>>$@
 	echo 'end lib$(name);'					>>$@
 
-$(obj)/gnatprove/gnatprove.out: $(gpr) $$($(name)-gens) $(obj)/proofmode
+$(obj)/gnatprove/gnatprove.out: $(gpr) $$($(name)-srcs) $$($(name)-gens) $(obj)/proofmode
 	gnatprove -P$< $(SPARKFLAGS) -Xobj=$(abspath $(obj))
 
 proof: