SDK: correctly zip sources.zip package.

When building the sources.zip for the SDK Repository,
the --exec-zip argument seems to be have been lost with
recent merges. Without this flag, the sources.zip is
not valid on some platforms.

Change-Id: I589df3d6df7890e0fccf568a03a76e6985ecb534
diff --git a/build/tools/sdk_repo.mk b/build/tools/sdk_repo.mk
index 137a591..deb3bdc 100644
--- a/build/tools/sdk_repo.mk
+++ b/build/tools/sdk_repo.mk
@@ -92,7 +92,7 @@
 define mk-sdk-repo-sources
 $(call sdk-repo-pkg-zip,$(1),$(2),$(3)): $(2) $(HOST_OUT)/development/sdk/source_source.properties
 	@echo "Building SDK sources package"
-	$(hide) $(TOPDIR)development/build/tools/mk_sources_zip.py \
+	$(hide) $(TOPDIR)development/build/tools/mk_sources_zip.py --exec-zip \
 			$(HOST_OUT)/development/sdk/source_source.properties \
 			$(call sdk-repo-pkg-zip,$(1),$(2),$(3)) \
 			$(TOPDIR).