Update for extern unsafe fn changes

    $ rust -v
    rustc 0.11-pre (adb8b0b 2014-05-10 23:21:44 -0700)
    host: x86_64-unknown-linux-gnu
    $ make
    rustc -O --cfg ndebug  --dep-info build/openssl.d --out-dir build lib.rs
    crypto/pkey.rs:92:36: 92:42 error: expected `fn`, found `unsafe`
    crypto/pkey.rs:92     fn _tostr(&self, f: extern "C" unsafe fn(*EVP_PKEY, **mut u8) -> c_int) -> Vec<u8> {
                                                         ^~~~~~
    Makefile:20: recipe for target 'build/libopenssl-06cd96fd-0.0.so' failed
    make: *** [build/libopenssl-06cd96fd-0.0.so] Error 101