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.<plus>. 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 28f0498.

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

Author: MasaoFujii

src/interfaces/ecpg/preproc/meson.build

@@ -96,6 +96,6 @@ tests += {
       't/001_ecpg_err_warn_msg.pl',
       't/002_ecpg_err_warn_msg_informix.pl',
     ],
-    'deps': ecpg_exe,
+    'deps': [ecpg_exe],
   },
 }