pgsql: Add missing Int64GetDatum macro in dbsize.c

Add missing Int64GetDatum macro in dbsize.c

I accidentally missed adding this when adjusting 55fe60938 for back
patching.  This adjustment was made for 9.6 to 13. 14 and master are not
affected.

Discussion: https://postgr.es/m/CAApHDvp=twCsGAGQG=A=cqOaj4mpknPBW-EZB-sd+5ZS5gCTtA@mail.gmail.com

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/dd76aa54a3cf50e6b53b99c03ed008a15d6a7c35

Modified Files
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src/backend/utils/adt/dbsize.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)