Add dependencies from the def files to the libraries that generate them

Add dependencies from the def files to the libraries that generate them

This avoids build failures in parallel builds, where builds
could fail with this error message:

make[2]: *** No rule to make target `libgnutls-26.def', needed by `all-am'.  Stop.

There is no direct rules that generate it, but it is generated
as a byproduct when building libgnutls.la. By marking the la
file as a dependency, make won't bail out by not finding the file
until that dependency is built, and at that point, the def file
exists.

Signed-off-by: Simon Josefsson <simon@josefsson.org>