build-sys: Always prepend '-' to BUILDID
    
    This was removed by commit 28a6bd6 as WINDOWS_PRODUCTVERSION
    needs a buildid without a dash. Apart from this variable,
    all other uses of buildid/BUILDID in virt-viewer source tree
    need a dash between the version number and the buildid to avoid getting
    output like "3.01" instead of "3.0-1"
    
    Rather than patching every location where BUILDID is used, this commit
    appends the "-" before substituting/defining BUILDID in configure.ac.
    This does not modifies the buildid configure.ac variable, this way
    WINDOWS_PRODUCTVERSION won't get an unwanted '-'.