Fix confusing indentations which trigger GCC >=6.x warnings

[Eyal Rozenberg]

When building with GCC 6.x or 7.x, we get warnings about confusing indentation, e.g.:

/path/to/tpch-dbgen/bm_utils.c:417:5: warning: this ‘for’ clause does not guard... [-Wmisleading-indentation]
     for (; n > 0; ++s1, ++s2, --n)
     ^~~
/path/to/tpch-dbgen/bm_utils.c:422:9: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘for’
         return (0);
         ^~~~~~

Now, the use of whitespace in the source files is generally "off" and messy, even interspersing some occasional tabs for good measure, but let's at least fix these lines specifically.

[Eyal Rozenberg]

Note the subsequent issue #19.