testing/postgresql-pg_variables: fix build error for postgresql-pg_variables

postgresql-pg_variables build error when run abuild -r,the log as follow:

pg_variables.c:1585:2: error: call to undeclared function 'AssertArg'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration] AssertArg(MemoryContextIsValid(context));