systemd: run libgcrypt-config from cross-compile enviroment

This makes configure using libgcrypt-config from cross-compile enviroment
instead of native enviroment.

Signed-off-by: Kalle Lampila <kalle.lampila@ixonos.com>
Please replace with --with-libgcrypt-prefix=$SYSROOT/usr

$SYSROOT automatically expands into the cross or native sysroot depending on which type of build is processed.

Use of NATIVE_SYSROOT and CROSS_SYSROOT is only needed when cross type buildfiles need to refer into native sysroot and vice versa.