Silence -Werror,-Wunused-private-field

...with !USE_CRYPTO_ANY (e.g., Android)

Change-Id: I139aceeea085420dd3ec31ced25b66a1671dc7ad
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/178130
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <stephan.bergmann@allotropia.de>

svl/source/crypto/cryptosign.cxx

@@ -936,6 +936,7 @@ bool Signing::Sign(OStringBuffer& rCMSHexBuffer)
 {
 #if !USE_CRYPTO_ANY
     (void)rCMSHexBuffer;
+    (void)m_rSigningContext;
     return false;
 #else
     // Create the PKCS#7 object.