security/libsecret: fix build by accounting for cmsgcred

The relevant errored code comes directly from security/gnome-keyring,
which builds successfully there because its configure process
properly detects and accounts for cmsgcred in <sys/socket.h>. In
porting the code here, the relevant configure checks did not make
it in, so the build goes down a code path not intended for us,
hitting the error. Temporarily define the cmsgcred preprocessor
variable as part of MESON_ARGS until upstream takes appropriate
action in meson.build.

Reported by: Ale <discipline[at]tiscali[dot]it>
PR: 287429