fix Makefiles to include SIGC_CFLAGS

now that the framebuffer class includes libsigc++, the path
to it must be known to all parts using framebuffer.h

lib/libdvbsub/Makefile.am

@@ -4,6 +4,7 @@ AM_CPPFLAGS = \
 	-I$(top_srcdir)/src \
 	-I$(top_srcdir)/src/zapit/include \
 	@AVFORMAT_CFLAGS@ \
+	@SIGC_CFLAGS@ \
 	@HWLIB_CFLAGS@
 
 AM_CPPFLAGS += -fno-rtti -fno-exceptions