build: workaround broken SASL header (again)
    
    Compilation for xdg-app failed due to a buggy SASL headers present on
    the used runtime (org.gnome.Sdk 3.18).
    
    In file included from rpc/virnetsaslcontext.h:24:0,
                     from rpc/virnetsaslcontext.c:25:
    /usr/include/sasl/sasl.h:230:38: error: unknown type name 'size_t'
     typedef void *sasl_realloc_t(void *, size_t);
                                          ^
    /usr/include/sasl/sasl.h:235:5: error: unknown type name 'sasl_realloc_t'
         sasl_realloc_t *,
    
    Use the same workaround as commit 1be3dfd did.
    
    Signed-off-by: Ján Tomko <jtomko@redhat.com>