Fix a number of style nits in the spillbuf implementation.

* subversion/include/private/svn_subr_private.h,
  subversion/libsvn_subr/spillbuf.c,
  subversion/tests/libsvn_subr/spillbuf-test.c: According to our style guide
   in declarations * binds to the name, not the type of the variable.

Found by: gstein


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1509153 13f79535-47bb-0310-9956-ffa450edef68