src/borg/crypto/low_level.pyx: fix compiler warning

The generated source code was producing a compiler warning due to
the pointers differing in constness. The called function expects
a non-const pointer while the generated code produces a const pointer
via a cast. This changes the cast to drop 'const' to make the compiler
happy.

src/borg/crypto/low_level.pyx

@@ -580,7 +580,7 @@ cdef class _AEAD_BASE:
                 raise CryptoError('EVP_CIPHER_CTX_ctrl SET IVLEN failed')
             if not EVP_DecryptInit_ex(self.ctx, NULL, NULL, self.enc_key, iv):
                 raise CryptoError('EVP_DecryptInit_ex failed')
-            if not EVP_CIPHER_CTX_ctrl(self.ctx, EVP_CTRL_GCM_SET_TAG, self.mac_len, <const unsigned char *> idata.buf+hlen):
+            if not EVP_CIPHER_CTX_ctrl(self.ctx, EVP_CTRL_GCM_SET_TAG, self.mac_len, <unsigned char *> idata.buf+hlen):
                 raise CryptoError('EVP_CIPHER_CTX_ctrl SET TAG failed')
             rc = EVP_DecryptUpdate(self.ctx, NULL, &olen, <const unsigned char*> idata.buf+aoffset, alen)
             if not rc: