adjust include directories for building observation

Author: phausler

stdlib/public/Observation/Sources/Observation/CMakeLists.txt

@@ -14,6 +14,8 @@ set(SWIFT_OBSERVATION_SWIFT_FLAGS)
 
 set(SWIFT_HOST_LIBRARIES_SUBDIRECTORY "swift/host")
 
+list(APPEND swift_runtime_library_compile_flags  -I${SWIFT_SOURCE_DIR}/include)
+
 list(APPEND SWIFT_OBSERVATION_SWIFT_FLAGS
   "-enable-experimental-feature" "Macros"
   "-load-plugin-library"