The shadowsocks source uses the name `encrypt` which is already reserved by
the `unistd.h` header. Rename the local `encrypt` and `decrypt` functions to
`crypto_encrypt` and `crypto_decrypt` in order to prevent clashes with the
standard headers.

Fixes the following build error oberserved on the buildbot:

    In file included from client.c:19:0:
    crypto.h:18:5: error: conflicting types for 'encrypt'
     int encrypt(int sockfd, struct link *ln);
         ^
    In file included from .../staging_dir/toolchain-mipsel_mips32_gcc-4.8-linaro_musl-1.1.10/include/fortify/unistd.h:20:0,
                     from client.c:12:
    .../staging_dir/toolchain-mipsel_mips32_gcc-4.8-linaro_musl-1.1.10/include/unistd.h:145:6: note: previous declaration of 'encrypt' was here
     void encrypt(char *, int);
          ^
    make[3]: *** [sslocal] Error 1

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>