pgsql: Use standard librart sqrt function in pg_stat_statements

Use standard librart sqrt function in pg_stat_statements

The stddev calculation included a faster but unportable sqrt function.
This is not worth the extra effort, and won't work everywhere. If the
standard library function is good enough for the SQL function it
should be good enough here too.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/cfe12763c32437bc708a64ce88a90c7544f16185

Modified Files
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contrib/pg_stat_statements/pg_stat_statements.c |   21 ++-------------------
1 file changed, 2 insertions(+), 19 deletions(-)