fatfs: fix unit tests not included in the build

Regression from 9baa7826.

components/fatfs/test/CMakeLists.txt

@@ -3,7 +3,7 @@ if(IDF_TARGET STREQUAL "esp32")
     list(APPEND srcdirs "esp32")
 endif()
 
-idf_component_register(SRC_DIRS ${src_dirs}
+idf_component_register(SRC_DIRS ${srcdirs}
                        INCLUDE_DIRS .
                        REQUIRES unity test_utils vfs fatfs
                        EMBED_TXTFILES fatfs.img