Remove include_directories() from cmake `mlir_tablegen()` function (NFC)

This is present since the beginning, but does not seem needed by any
in-tree target right now. This seems like the kind of thing to populate
by the caller if needed.

Differential Revision: https://reviews.llvm.org/D121565

mlir/cmake/modules/AddMLIR.cmake

@@ -5,7 +5,6 @@ function(mlir_tablegen ofn)
   tablegen(MLIR ${ARGV})
   set(TABLEGEN_OUTPUT ${TABLEGEN_OUTPUT} ${CMAKE_CURRENT_BINARY_DIR}/${ofn}
       PARENT_SCOPE)
-  include_directories(${CMAKE_CURRENT_BINARY_DIR})
 endfunction()
 
 # Declare a dialect in the include directory