ra_serf: Fix compilation error when gssapi is not installed.
ra_serf: Fix compilation error when gssapi is not installed.

* subversion/libsvn_ra_serf/auth_kerb.c
  (global): Move the test for SVN_RA_SERF_GSSAPI_ENABLED before 
   including <gssapi.h>.

Found by: arfrever

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion