Fix -Wmissing-include-dirs warnings

Fix -Wmissing-include-dirs warnings

We get this warning:

      CC       details.lo
    cc1: error: /home/smarchi/src/babeltrace/plugins: No such file or directory [-Werror=missing-include-dirs]

And indeed, src/plugins/text/details/Makefile.am sets a -I pointing to a
directory that doesn't exist anymore, so just remove it.

Change-Id: Id9b8ba81cc671c0ba862b47ff337fde07f8d4c1d
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2261
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>