The recipe google-authenticator-libpam_1.09.bb does not actually install the file pam_google_authenticator.so to path /lib/security, in fact it is not installed at all. The binary /usr/bin/google-authenticator is installed, although it is not mentionend in the FILES list.

I think the problem is that the recipe defines a new package:
PACKAGES += "pam-google-authenticator"
but somehow the files of that package are not installed.

I managed to fix it, maybe someone with more knowledge can take a look at this.
Improvement idea: It would be nice if the installation of the binary was optional and non-default.
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SUMMARY = "Google Authenticator PAM module"
HOME_PAGE = "https://github.com/google/google-authenticator-libpam"
LIC_FILES_CHKSUM = "file://LICENSE;md5=3b83ef96387f14655fc854ddc3c6bd57"
LICENSE = "Apache-2.0"

SRC_URI = "git://github.com/google/google-authenticator-libpam.git;branch=master;protocol=https"
SRCREV = "962f353aac6cfc7b804547319db40f8b804f0b6c"

DEPENDS = "libpam"

S = "${WORKDIR}/git"

inherit autotools features_check

REQUIRED_DISTRO_FEATURES = "pam"

# Use the same dir location as PAM
EXTRA_OECONF = "--libdir=${base_libdir}"
FILES:${PN} += "${base_libdir}/security/pam_google_authenticator.so"
FILES:${PN} += "${bindir}/google-authenticator"

RDEPENDS_${PN}  = "libpam"