Add FFTW3 to PUBLIC target_include_dirs

So that it will be inherited by the app targets

src/core/CMakeLists.txt

@@ -126,6 +126,7 @@ target_include_directories(
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/stager>
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/scatter_devices>
   $<BUILD_INTERFACE:${CMAKE_BINARY_DIR}/generated>
+  ${FFTW3_INCLUDE_DIR}
   PRIVATE
   ${CMAKE_CURRENT_SOURCE_DIR}
   ${CMAKE_CURRENT_SOURCE_DIR}/math