pgsql: Fix incorrect file reference in guc.h

Fix incorrect file reference in guc.h

GucSource_Names was documented as being in guc.c, but since 0a20ff54f5e6
it is located in guc_tables.c.  The reference to the location of
GucSource_Names is important, as GucSource needs to be kept in sync with
GucSource_Names.

Author: David G. Johnston <david.g.johnston@gmail.com>
Discussion: https://postgr.es/m/CAKFQuwYPgAHWPYjPzK7iXzhSZ6MKR8w20_Nz7ZXpOvx=kZbs7A@mail.gmail.com
Backpatch-through: 16

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c88ce73eda2e0a818d730c5b72475ef99cc9c4cf

Modified Files
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src/include/utils/guc.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)