This shouldn't be necessary, that's why we have one extra patch to make the includes consistent,
https://src.fedoraproject.org/rpms/plasma-desktop/blob/master/f/plasma-desktop-5.18.4.1-synaptics_includes.patch

if you take a close look at the spec file's build section you'll see this:

upstreamable patches

%patch200 -p1

%if ! 0%{?synaptics}

install -pD %{SOURCE200} 3rdparty/xorg/synaptics-properties.h

%endif

%patch201 -p1 -b .synaptics_includes

Martin commented out the patches and copies the include file to 3rdparty/xorg instead. That restults in a build error as the include path is then omitted wrong:

%{cmake_kf5} .. \

%{?!synaptics:-DSynaptics_INCLUDE_DIRS:PATH="$(pwd)/../3rdparty"}

where the xorg is missing in the path. The source file expects <synaptics-properties.h>.

And that's precisely what plasma-desktop-5.18.4.1-synaptics_includes.patch fixes:

-#include <synaptics-properties.h>
+#include <xorg/synaptics-properties.h>

I'm perfectly fine with any solution as long as it fixes the build problem correctly :)

Now we have a mix of two solutions where I don't know why Martin commented out the patch that was there before - it didn't work for me though either.

We need patch200 applied too, I'll fix this slightly differently.