Though no error was raised during a native build, a cross-build was
choking on this missing link to libgimpbase and failing.
The error returned by the linker though was a bit amiss.

Fixes:
> gimp-test-clipboard.o: In function `test_clipboard_copy_callback':
> tools/gimp-test-clipboard.c:419: undefined reference to `g_file_get_contents'
> collect2: error: ld returned 1 exit status