sasl: Only define Curl_sasl_digest_get_pair() when CRYPTO_AUTH enabled

Introduced in commit 59f3f92b this function is only implemented when
CURL_DISABLE_CRYPTO_AUTH is not defined. As such we shouldn't define
the function in the header file either.

lib/curl_sasl.h

@@ -141,15 +141,14 @@ char *Curl_sasl_build_spn(const char *service, const char *instance);
 TCHAR *Curl_sasl_build_spn(const char *service, const char *instance);
 #endif
 
-/* This is used to extract the realm from a challenge message */
-int Curl_sasl_digest_get_pair(const char *str, char *value, char *content,
-                              const char **endptr);
-
 #if defined(HAVE_GSSAPI)
 char *Curl_sasl_build_gssapi_spn(const char *service, const char *instance);
 #endif
 
 #ifndef CURL_DISABLE_CRYPTO_AUTH
+/* This is used to extract the realm from a challenge message */
+int Curl_sasl_digest_get_pair(const char *str, char *value, char *content,
+                              const char **endptr);
 
 /* This is used to generate a base64 encoded DIGEST-MD5 response message */
 CURLcode Curl_sasl_create_digest_md5_message(struct SessionHandle *data,