Makefile.vc: fix webp_quality.exe link

when building a dll based libwebp include the dsp private symbols that
WebPUnmultiplyARGB requires

Change-Id: I7cf7da0b20d6cf6740219c8562380926a0abd93c
(cherry picked from commit cf047e8)

Makefile.vc

@@ -391,7 +391,13 @@ $(DIRBIN)\get_disto.exe: $(IMAGEIO_DEC_OBJS) $(IMAGEIO_UTIL_OBJS)
 $(DIRBIN)\get_disto.exe: $(LIBWEBPDEMUX) $(LIBWEBP)
 $(DIRBIN)\webp_quality.exe: $(DIROBJ)\extras\webp_quality.obj
 $(DIRBIN)\webp_quality.exe: $(IMAGEIO_UTIL_OBJS)
-$(DIRBIN)\webp_quality.exe: $(EXTRAS_OBJS) $(LIBWEBP)
+$(DIRBIN)\webp_quality.exe: $(EXTRAS_OBJS)
+# EXTRA_OBJS requires private symbols from dsp. Explicitly add those when
+# building libwebp as a dll.
+!IF "$(DLLBUILD)" == "TRUE"
+$(DIRBIN)\webp_quality.exe: $(DSP_DEC_OBJS)
+!ENDIF
+$(DIRBIN)\webp_quality.exe: $(LIBWEBP)
 $(DIRBIN)\webpinfo.exe: $(DIROBJ)\examples\webpinfo.obj
 $(DIRBIN)\webpinfo.exe: $(IMAGEIO_DEC_OBJS)
 $(DIRBIN)\webpinfo.exe: $(EX_UTIL_OBJS) $(IMAGEIO_UTIL_OBJS)