Fix variable declaration with no type in the configure script.

We recently attempted to turn several warnings into errors in Gecko (see [bug 1076698](https://bugzilla.mozilla.org/show_bug.cgi?id=1076698)). However, the checks made in jemalloc's configure script to determine the version of the malloc zone API in OSX trigger -Wimplicit-int, which breaks the build in that platform (I filed [bug 1080851](https://bugzilla.mozilla.org/show_bug.cgi?id=1080851) for that). The patch in this PR properly declares the `foo` vector in the configure script, thus silencing the warning.