omx: fix autotools build for generic target
omx: fix autotools build for generic target

gstomxvideoenc.c:2874:7: error: "USE_OMX_TARGET_ZYNQ_USCALE_PLUS" is not defined, evaluates to 0 [-Werror=undef]
 #elif USE_OMX_TARGET_ZYNQ_USCALE_PLUS

Works on meson because it doesn't use -Wundef

GStreamer/gst-omx,GStreamer/gst-omx,GStreamer/gst-omx