Fix autotools build
commit 6ce69859:
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
commit 6b292927:
meson: add -Wundef as additional warning flag