lib/libdmsg: Fix compile warnings on Linux distros

lib/libdmsg: Fix compile warnings on Linux distros

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crypto.c: In function 'dmsg_crypto_negotiate':
crypto.c:552: warning: pointer targets in passing argument 2 of 'RSA_private_encrypt' differ in signedness
/usr/include/openssl/rsa.h:305: note: expected 'const unsigned char *' but argument is of type 'char *'
crypto.c:552: warning: pointer targets in passing argument 3 of 'RSA_private_encrypt' differ in signedness
/usr/include/openssl/rsa.h:305: note: expected 'unsigned char *' but argument is of type 'char *'