Use prefix to determine pam_modules_dir
If prefix=/usr and libdir=lib64, then by default, this package will install the pam module under /lib64/security, disregarding /usr. Prepend get_option('prefix')
to the path in pam/meson.build consistent with its use in meson.build.
/lib64/security/pam_fprintd.so
-> /usr/lib64/security/pam_fprintd.so
Fixes #172 (closed)