shared/cryptsetup-util: build problematic code only in developer mode

shared/cryptsetup-util: build problematic code only in developer mode

This code doesn't link when gcc+lld is used:

$ LDFLAGS=-fuse-ld=lld meson setup build-lld && ninja -C build-lld udevadm
...
ld.lld: error: src/shared/libsystemd-shared-255.a(libsystemd-shared-255.a.p/cryptsetup-util.c.o):
  symbol crypt_token_external_path@@ has undefined version
collect2: error: ld returned 1 exit status

As a work-around, restrict it to developer mode.

Closes https://github.com/systemd/systemd/issues/30218.