style: remove extra const keyword from interfaces

Const variables in prototypes don't add much value, but make the code
larger and redundant. Remove these const keywords.
Note, this doesn't apply to pointers to const memory.

src/tls_schannel.c

@@ -412,7 +412,7 @@ int tls_pending(tls_t *tls)
     return 0;
 }
 
-int tls_read(tls_t *tls, void *const buff, const size_t len)
+int tls_read(tls_t *tls, void *buff, size_t len)
 {
     int bytes;
 
@@ -557,7 +557,7 @@ int tls_clear_pending_write(tls_t *tls)
     return 1;
 }
 
-int tls_write(tls_t *tls, const void *const buff, const size_t len)
+int tls_write(tls_t *tls, const void *buff, size_t len)
 {
     SecBufferDesc sbdenc;
     SecBuffer sbenc[4];