Read this for how to use the rdesktop patch:

This patch is intended for rdesktop 1.3.1 only. It has not been tested
with other versions, and future versions should (hopefully) include it
by default.

- Get a copy of the rdesktop 1.3.1 sources and unpack it.
- Place the rdesktop.patch file in the top directory of the unpacked
  sources.
- Go to the directory where you placed the patch and type the following
  command: patch -p 0 -i rdesktop.patch
- You can now compile the sources as usual.