cmake: set UMEM_{LIBRARY,INCLUDE_DIR} to the empty string
cmake: set UMEM_{LIBRARY,INCLUDE_DIR} to the empty string

Without providing an explicit value, the NOT_FOUND is preserved causing
build failures.

Signed-off-by: Josef 'Jeff' Sipek <a620f141433c70aa9e43778305a6c68cbfadfb25@josefsipek.net>

jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc