rgw: add -ldl for mongoose

/usr/bin/ld: mongoose/mongoose.o: undefined reference to symbol 'dlsym@@GLIBC_2.2.5'
/lib/x86_64-linux-gnu/libdl.so.2: error adding symbols: DSO missing from command line
error: collect2: ld returned 1 exit status

Signed-off-by: Sage Weil <sage@inktank.com>