Follow-up to r1932040: Fix include of private/svn_atomic.h header file.

* subversion/libsvn_subr/checksum_xdigest.c
  (includes): Use h-char ("header.h") syntax instead of q-char (<header.h>) to
   include private/svn_atomic.h.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1932138 13f79535-47bb-0310-9956-ffa450edef68