Fix cmake flag typo.

[jlefevre]

Fixes small typo (cmake flag) in popper.yml

Previously was getting cmake warning:

-- Disabling Kafka endpoint support
-- exclude following files under src: *.js;*.css;civetweb;erasure-code/jerasure/jerasure;erasure-code/jerasure/gf-complete;rocksdb;googletest;spdk;xxHash;isa-l;lua;zstd;crypto/isa-l/isa-l_crypto;blkin;rapidjson;dmclock;seastar;c-ares
-- Configuring done
-- Generating done
CMake Warning:
  Manually-specified variables were not used by the project:

    DCMAKE_BUILD_TYPE

-- Build files have been written to: /workspace/ceph/build
Scanning dependencies of target Boost
Creating directories for 'Boost'
Performing download step (download, verify and extract) for 'Boost'
-- Downloading...

now fixed:

-- Disabling Kafka endpoint support
-- exclude following files under src: *.js;*.css;civetweb;erasure-code/jerasure/jerasure;erasure-code/jerasure/gf-complete;rocksdb;googletest;spdk;xxHash;isa-l;lua;zstd;crypto/isa-l/isa-l_crypto;blkin;rapidjson;dmclock;seastar;c-ares
-- Configuring done
-- Generating done
-- Build files have been written to: /workspace/ceph/build
Scanning dependencies of target Boost
Creating directories for 'Boost'
Performing download step (download, verify and extract) for 'Boost'
-- Downloading...