libswscale is needed in Cygwin vhooks

patch by Víctor Paesa wzrlpy arsystel com

Originally committed as revision 8013 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1053,8 +1053,8 @@ EOF
     targetos=CYGWIN
     shlibdir="$bindir"
     dv1394="no"
-    VHOOKSHFLAGS='-shared -L$(BUILD_ROOT)/libavformat -L$(BUILD_ROOT)/libavcodec -L$(BUILD_ROOT)/libavutil'
-    VHOOKLIBS='-lavformat$(BUILDSUF) -lavcodec$(BUILDSUF) -lavutil$(BUILDSUF) $(EXTRALIBS)'
+    VHOOKSHFLAGS='-shared -L$(BUILD_ROOT)/libavformat -L$(BUILD_ROOT)/libavcodec -L$(BUILD_ROOT)/libswscale -L$(BUILD_ROOT)/libavutil'
+    VHOOKLIBS='-lavformat$(BUILDSUF) -lavcodec$(BUILDSUF) -lswscale$(BUILDSUF) -lavutil$(BUILDSUF) $(EXTRALIBS)'
     osextralibs=""
     EXESUF=".exe"
     SLIBPREF="cyg"