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