cmake: link against ZLIB if statically linked agasint Boost

cmake: link against ZLIB if statically linked agasint Boost

Boost::iostreams links against zlib. and FindBoost.cmake
does not add this linkage to Boost::iostreams target, let's
do this after `find_package(Boost...)`. in theory, it'd be
better to have this change in FindBoost.cmake, but it's
error-prone, and increases the risk of regression when
we sync our own copy of FindBoost.cmake with CMake upstream.

../../build.deps/mingw/boost/lib/libboost_iostreams.a(zlib.o):zlib.cpp:(.text+0xf7): undefined reference to `crc32'
../../build.deps/mingw/boost/lib/libboost_iostreams.a(zlib.o):zlib.cpp:(.text+0x165): undefined reference to
`deflateReset'../../build.deps/mingw/boost/lib/libboost_iostreams.a(zlib.o):zlib.cpp:(.text+0x185): undefined reference to `inflateEnd'
../../build.deps/mingw/boost/lib/libboost_iostreams.a(zlib.o):zlib.cpp:(.text+0x1a1): undefined reference to
`inflateReset'../../build.deps/mingw/boost/lib/libboost_iostreams.a(zlib.o):zlib.cpp:(.text+0x1b3): undefined reference to `deflateEnd'
../../build.deps/mingw/boost/lib/libboost_iostreams.a(zlib.o):zlib.cpp:(.text+0x38d): undefined reference to `inflateInit2_'
../../build.deps/mingw/boost/lib/libboost_iostreams.a(zlib.o):zlib.cpp:(.text+0x3d5): undefined reference to `deflateInit2_'
../../build.deps/mingw/boost/lib/libboost_iostreams.a(zlib.o):zlib.cpp:(.text+0x134): undefined reference to `deflate'
../../build.deps/mingw/boost/lib/libboost_iostreams.a(zlib.o):zlib.cpp:(.text+0x144): undefined reference to `inflate'
collect2: error: ld returned 1 exit status
src/CMakeFiles/ceph-common.dir/build.make:441: recipe for target 'bin/libceph-common.dll' failed

Signed-off-by: Kefu Chai <kchai@redhat.com>