libs.mk: Expose RC_RTC_SRCS as libvpxrc_srcs.txt

This allows other build systems to get a list of the source files
needed to build the rate-control library.

Change-Id: Id9228e024736bd7a888f8b66db06b86243cac7f7

libs.mk

@@ -214,6 +214,11 @@ libvpx_srcs.txt:
 	@echo $(CODEC_SRCS) | xargs -n1 echo | LC_ALL=C sort -u > $@
 CLEAN-OBJS += libvpx_srcs.txt
 
+libvpxrc_srcs.txt:
+	@echo "    [CREATE] $@"
+	@echo $(RC_RTC_SRCS) | xargs -n1 echo | LC_ALL=C sort -u > $@
+CLEAN-OBJS += libvpxrc_srcs.txt
+
 # Assembly files that are included, but don't define symbols themselves.
 # Filtered out to avoid Windows build warnings.
 ASM_INCLUDES := \