tools.mk: Expose tiny_ssim.SRCS as tiny_ssim_srcs.txt

This CL adds the change to get the source files to build the tiny_ssim
binary.

Bug: 410594665
Change-Id: I3679b22c6b68130e5f874fbecb188fa2f28eed67
(cherry picked from commit 748eb8338906a21e1a68d6aaf1c17af345140e1b)

Change-Id: I3679b22c6b68130e5f874fbecb188fa2f28eed67
diff --git a/README.android b/README.android
index 26d2a77..704e49b 100644
--- a/README.android
+++ b/README.android
@@ -7,6 +7,7 @@
 24b568e6f libs.mk: Expose RC_RTC_SRCS as libvpxrc_srcs.txt
 7e78c7169 examples.mk: Expose vpxdec.SRCS as vpxdec_srcs.txt
 b2a919773 Fix test VP8/9 encoder CTS failure
+748eb8338 tools.mk: Expose tiny_ssim.SRCS as tiny_ssim_srcs.txt
 
 Description:
 Contains the sources used to compile libvpx.
diff --git a/tools.mk b/tools.mk
index dd2ebeb..79bb0cb 100644
--- a/tools.mk
+++ b/tools.mk
@@ -91,6 +91,13 @@
 $(foreach proj,$(call enabled,PROJECTS),\
     $(eval $(call vcproj_template,$(proj))))
 
+# Generate a list of all enabled sources, in particular for exporting to gyp
+# based build systems.
+tiny_ssim_srcs.txt:
+	@echo "    [CREATE] $@"
+	@echo $(tiny_ssim.SRCS) | xargs -n1 echo | LC_ALL=C sort -u > $@
+CLEAN-OBJS += tiny_ssim_srcs.txt
+
 #
 # Documentation Rules
 #