Per this GitHub comment, the #ifdef MONGOC_ENABLE_SASL_SSPI guard should be moved above the private header includes, as is done for other C files with conditional code (e.g. mongoc-cluster-sasl.c). As-is, it's possible the trigger the following build error when including this C file when SSPI was not found (i.e. MONGOC_ENABLE_SASL_SSPI is undefined):

src/mongoc/mongoc-sspi-private.h:32:21: fatal error: Windows.h: No such file or directory