I think @docdoc is onto something.
When you compile a sketch, Arduino IDE searches for the library that provides each of the header files specified in the #include directives of your sketch program's code (both the code in the sketch itself, and in the libraries used by the sketch). It then adds the paths of the discovered libraries to the compiler's "search path". However, this "library discovery" system only searches the root of the library source folders, so it won't discover libraries for which the #include directive specifies a header file in a subfolder of the library.
So the way to fix this problem is to cause Arduino IDE to discover the "rmtNEO" library that provides the driver/rmt_rx.h header file, making sure the discovery occurs prior to reaching the #include directive for the header file in the subfolder of the library. You can do that by adding an #include directive for a header file from the source root folder of the library above the #include directive for DShotRMT.h (which transitively provides the #include directive for driver/rmt_rx.h).
So add the following code to the top of your sketch:
#include <clk_tree_defs.h>
Then compile the sketch again. When I did that, I found that the "driver/rmt_rx.h: No such file or directory" error no longer occurred. I did encounter an new error:
C:\Users\per\Documents\Arduino\sketch_jun7a\sketch_jun7a.ino:5:24: error: 'RMT_CHANNEL_0' was not declared in this scope
DShotRMT motorFL(p_FL, RMT_CHANNEL_0);
^~~~~~~~~~~~~
Using library rmtNEO at version 0.0.3 in folder: C:\Users\per\Documents\Arduino\libraries\rmtNEO
Using library DShot-rmt at version 4.3.0 in folder: C:\Users\per\Documents\Arduino\libraries\DShot-rmt
but at least it gets you past the library discovery problem.