Deletes unused variable 'struct RArray *sv' in struct.c .

Enviroment
OS : Mac OS X 10.8.2
Changes toolchain from gcc to clang.
<pre>build_config.rb
  toolchain :clang</pre>

Causes a warning when mruby is built.
<pre>
$ make
  ...
  CC    mrbgems/mruby-struct/src/struct.c -> build/host/mrbgems/mruby-struct/src/struct.o
  mrbgems/mruby-struct//src/struct.c:509:18: warning: unused variable 'sv' [-Wunused-variable]
  struct RArray *sv;
</pre>

Deletes unused variable 'struct RArray *sv'.
<pre>mrbgems/mruby-struct/src/struct.c
  struct RArray *sv
</pre>

Causes no warning when mruby is built.