build: workaround broken SASL header (again)

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>