The upstream author observed that some of the values he uses are not in the libssh2 public headers. I don't know where he uses them from, but I had to replace them here to get this to compile.
https://github.com/libssh2/libssh2/issues/90
diff -x '*.orig' -x '*.swp' -x '*.rej' -Naur 10.9-0/FreeFileSync/Source/fs/sftp.cpp 10.9-1/FreeFileSync/Source/fs/sftp.cpp
--- 10.9-0/FreeFileSync/Source/fs/sftp.cpp  2019-02-10 16:42:29.139040980 -0500
+++ 10.9-1/FreeFileSync/Source/fs/sftp.cpp  2019-02-10 21:28:00.030732089 -0500
@@ -56,9 +56,10 @@

 //attention: if operation fails due to time out, e.g. file copy, the cleanup code may hang, too => total delay = 2 x time out interval

-const size_t SFTP_OPTIMAL_BLOCK_SIZE_READ  = 4 * MAX_SFTP_READ_SIZE;     //https://github.com/libssh2/libssh2/issues/90
-const size_t SFTP_OPTIMAL_BLOCK_SIZE_WRITE = 4 * MAX_SFTP_OUTGOING_SIZE; //
-static_assert(MAX_SFTP_READ_SIZE == 30000 && MAX_SFTP_OUTGOING_SIZE == 30000, "reevaluate optimal block sizes if these constants change!");
+// these values are all manually derived from https://github.com/libssh2/libssh2/blob/master/src/sftp.h which for some weak reason are not included in the libssh2-devel headers.
+const size_t SFTP_OPTIMAL_BLOCK_SIZE_READ  = 4 * 30000;     //https://github.com/libssh2/libssh2/issues/90
+const size_t SFTP_OPTIMAL_BLOCK_SIZE_WRITE = 4 * 30000; //
+//static_assert(MAX_SFTP_READ_SIZE == 30000 && MAX_SFTP_OUTGOING_SIZE == 30000, "reevaluate optimal block sizes if these constants change!");
 /*
 Perf Test, Sourceforge frs, SFTP upload, compressed 25 MB test file: