You could try something like this…
FILE_LIST := $(wildcard $(LOCAL_PATH)/[DIRECTORY]/*.cpp)
LOCAL_SRC_FILES := $(FILE_LIST:$(LOCAL_PATH)/%=%)
… Change [DIRECTORY] to the actual directory of the files. If they are in the same directory as your .mk file then remove that part. Create the FILE_LIST variable to find all of the .cpp files under the [DIRECTORY] directory. Then use it in the file listing. The LOCAL_SRC_FILES line will then remove the LOCAL_PATH from the listing.