ecpg: Fix compiler warning in ecpg build with Meson.

ecpg: Fix compiler warning in ecpg build with Meson.

Previously, Meson could produce a warning about the use of 'deps' in ecpg:

    WARNING: Project targets '>=0.54' but uses a feature introduced in '0.60.0': list.. The right-hand operand was not a list.

The right-hand operand of 'deps' should be a list. This commit fixes
the warning by wrapping it with square brackets.

This issue was introduced in commit 28f04984f0c.

Author: Jacob Champion 
Discussion: https://postgr.es/m/CAOYmi+ks8wO06Ymxduw2h_eQJ_D4_jHGeyMK0P=p5Q3psnEdMA@mail.gmail.com